Sum of coefficients

Algebra Level 3

f ( 0 ) = 1 f ( 4 ) f ( 2 ) = 2 f ( 6 ) f ( 4 ) = 3 \begin{array} {rl} f(0) & = 1 \\ f(4) - f(2) & = 2\\ f(6) - f(4) & = 3\\ \end{array}

Given that f ( x ) f(x) is a quadratic polynomial satisfying the three equations above, what is the value of f ( 8 ) ? f(8)?


The answer is 11.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

65 solutions

By method of differences,as f ( x ) f(x) is a quadratic polynomial, the first difference line should be a arithmetic progression. Thus f ( 2 ) f ( 0 ) = 1 , f ( 8 ) f ( 6 ) = 4 f(2)-f(0)=1,f(8)-f(6)=4 . And finally f ( 8 ) = f ( 0 ) + [ f ( 2 ) f ( 0 ) ] + [ f ( 4 ) f ( 2 ) ] + [ f ( 6 ) f ( 4 ) ] + [ f ( 8 ) f ( 6 ) ] = 1 + 1 + 2 + 3 + 4 = 11 f(8)=f(0)+[f(2)-f(0)]+[f(4)-f(2)]+[f(6)-f(4)]+[f(8)-f(6)]=1+1+2+3+4=11

Moderator note:

This is a very nice solution using the Method of Differences. There is a slight misprint, see the comments.

Shouldn't it be [ f ( 8 ) f ( 6 ) ] [f(8) - f(6)] rather than f ( 8 ) f(8) ?

Tim Vermeulen - 7 years, 10 months ago

Log in to reply

yeah you're right, Ivan, Kai Chin, must have missed that, it will be [f(8) - f(6)] otherwise f(6) will not get cancelled

Deepansh Mathur - 7 years, 10 months ago

Nice solution !

Priyansh Sangule - 7 years, 10 months ago

this one's the best!

Deepansh Mathur - 7 years, 10 months ago
Steven Yang
Jul 24, 2013

We can assume f ( x ) f(x) takes on the form a x 2 + b x + c ax^2 + bx + c since it is quadratic. Since f ( 0 ) = 1 f(0) = 1 , we can deduce that c = 1 c = 1 . By substituting the values 2, 4, and 6 we get:

f ( 4 ) f ( 2 ) = 16 a + 4 b + 1 ( 4 a + 2 b + 1 ) = 12 a + 2 b = 2 f(4) - f(2) = 16a + 4b + 1 - (4a + 2b + 1) = 12a + 2b = 2 f ( 6 ) f ( 4 ) = 36 a + 6 b + 1 ( 16 a + 4 b + 1 ) = 20 a + 2 b = 3 f(6) - f(4) = 36a + 6b + 1 - (16a + 4b + 1) = 20a + 2b = 3

Given these two equations we can solve for a and b, which are 1 8 \frac{1}{8} and 1 4 \frac{1}{4} , respectively. Plugging these two values into the original quadratic expression, we arrive at f ( x ) = 1 8 x 2 + 1 4 x + 1 f(x) = \frac{1}{8}x^2 + \frac{1}{4}x + 1 . Thus f ( 8 ) = 8 + 2 + 1 = 11 f(8) = 8 + 2 + 1 = 11 .

Moderator note:

There have been many similar solutions, but this one is probably the best written one. It is one of the two mathematically different approaches to the problem, the other one being the Method of Differences.

Lily Ye
Jul 24, 2013

General form for quadratic function:

f ( x ) = a x 2 + b x + c f(x) = ax ^ {2} + bx + c

Using the first piece of given information:

f ( 0 ) = a ( 0 2 ) + b ( 0 ) + c = 1 f(0) = a (0 ^ {2}) + b (0) + c = 1

c = 1 c=1

Using second piece of given information:

f ( 4 ) f ( 2 ) = a ( 4 2 ) + b ( 4 ) + 1 ( a ( 2 2 ) + b ( 2 ) + 1 ) f(4) - f(2) = a(4 ^ {2}) + b (4) + 1 - (a (2 ^ {2}) + b (2) + 1)

f ( 4 ) f ( 2 ) = 12 a + 2 b = 2 f(4) - f(2) = 12 a + 2 b =2

Using third piece of given information:

f ( 6 ) f ( 4 ) = a ( 6 2 ) + b ( 6 ) + 1 ( a ( 4 2 ) + b ( 4 ) + 1 ) f(6) -f(4) = a(6 ^ {2}) + b (6) + 1 - (a (4 ^ {2}) + b (4) + 1)

f ( 6 ) f ( 4 ) = 20 a + 2 b = 3 f(6) -f(4) = 20 a + 2 b =3

Combine second and third parts:

12 a + 2 b = 2 12 a + 2 b =2

20 a + 2 b = 3 20 a + 2 b =3

8 a = 1 -8a = -1

a = 1 8 a = \frac{1}{8}

Substitute into one of the equations to find b b :

12 1 8 + 2 b = 2 12 \frac{1}{8} + 2 b =2

b = 1 4 b = \frac{1}{4}

Substitute a = 1 8 , b = 1 4 , c = 1 a = \frac{1}{8}, b = \frac{1}{4}, c=1 into f ( x ) = a x 2 + b x + c f(x) = ax ^ {2} + bx + c :

f ( x ) = 1 8 x 2 + 1 4 x + 1 f(x) = \frac{1}{8}x ^ {2} + \frac{1}{4}x + 1

f ( 8 ) = 1 8 ( 8 ) 2 + 1 4 ( 8 ) + 1 f(8) = \frac{1}{8}(8) ^ {2} + \frac{1}{4}(8) + 1

f ( 8 ) = 11 f(8) = 11

WeiHao Teo
Jul 23, 2013

Let g : R R g : \mathbb{R} \to \mathbb{R} be such that g ( x ) = f ( 2 + x ) f ( x ) g(x) = f(2+x) - f(x) . Since f f is a quadratic, g g (which is a difference function of f f ) is a linear. We are given g ( 2 ) = 2 g(2) = 2 and g ( 4 ) = 3 g(4) = 3 , and so g ( 0 ) = 1 g(0) = 1 and g ( 6 ) = 4 g(6) = 4 . This give us f ( 8 ) f ( 0 ) = g ( 6 ) + g ( 4 ) + g ( 2 ) + g ( 0 ) = 10 f(8) - f(0) = g(6) + g(4) + g(2) + g(0) = 10 , and so f ( 8 ) = 11 f(8) = 11 .

Unique take on the problem!

Bob Krueger - 7 years, 10 months ago

Nice approach!

Caio Pelicioni - 7 years, 10 months ago
Otto Bretscher
Apr 28, 2015

For a quadratic, f ( 8 ) f ( 0 ) 8 = f ( 6 ) f ( 2 ) 4 = f ( 4 ) \frac{f(8)-f(0)}{8}=\frac{f(6)-f(2)}{4}=f'(4) . (The difference quotient is the derivative at the midpoint.) Adding two of the given equations, we find that f ( 6 ) f ( 2 ) = 5 f(6)-f(2)=5 . Thus f ( 8 ) 1 8 = 5 4 \frac{f(8)-1}{8}=\frac{5}{4} and f ( 8 ) = 11. f(8)=\boxed{11.}

Bob Krueger
Jul 21, 2013

After looking over the Method of Differences , and using 2 n 2n instead of n n , we can create, initially, the following table

2 n f ( 2 n ) D 1 ( 2 n ) D 2 ( 2 n ) 0 1 2 2 4 3 6 \begin{array}{l|lll} 2n & f(2n) & D_1(2n) & D_2(2n) \\ \hline 0 & 1 & ~ & ~ \\ 2 & ~ & 2 & ~ \\ 4 & ~ & 3 & ~ \\ 6 & ~ & ~ & ~ \\ \end{array}

Recognize that, since we are dealing with a quadratic, D 2 D_2 is constant. Thus, the following can be deduced

2 n f ( 2 n ) D 1 ( 2 n ) D 2 ( 2 n ) 0 1 1 1 2 2 2 1 4 4 3 1 6 7 4 1 \begin{array}{l|lll} 2n & f(2n) & D_1(2n) & D_2(2n) \\ \hline 0 & 1 & 1 & 1 \\ 2 & 2 & 2 & 1 \\ 4 & 4 & 3 & 1 \\ 6 & 7 & 4 & 1 \\ \end{array}

Continuing to work backwards in the table, one can find f ( 8 ) = f ( 6 ) + D 1 ( 6 ) = 7 + 4 = 11 f(8)=f(6)+D_1(6)=7+4=11 .

Finally, someone else that actually used the method of differences. :)

Edit: the tables don't render for me though, do they for you?

Tim Vermeulen - 7 years, 10 months ago

Log in to reply

Sadly not. I don't know why. Perhaps the latex reader built into the site isn't competent enough?

When I saw that this, I immediately thought method of differences. It may not look quick and easy (like in your solution - rather lengthy), but it is if you know what you are doing.

Bob Krueger - 7 years, 10 months ago

Log in to reply

Calculating the answer on paper went rather quickly, but actually typing it out using rigorous arguments takes a while.

Tim Vermeulen - 7 years, 10 months ago

f ( 0 ) = a 0 2 + b 0 + c f(0) = a*0^2 + b*0 + c

c = 1 \Rightarrow c = 1

f ( 4 ) f ( 2 ) = a 4 2 + b 4 + c a 2 2 b 2 c f(4) - f(2) = a*4^2 + b*4 + c - a*2^2 - b*2 - c

\Rightarrow 2 = 12a + 2b

f ( 6 ) f ( 4 ) = a 6 2 + b 6 + c a 4 2 b 4 c f(6) - f(4) = a*6^2 + b*6 + c - a*4^2 - b*4 - c

\Rightarrow 3 = 20a + 2b

3 2 = 20 a + 2 b 12 a 2 b 3 - 2 = 20a + 2b - 12a - 2b a = 1 8 \Rightarrow a = \frac{1}{8}

b = 1 6 a b = 1 - 6*a b = 1 6 1 8 \Rightarrow b = 1 - 6*\frac{1}{8} b = 1 4 \Rightarrow b = \frac{1}{4}

.

f ( 8 ) = a 8 2 + b 8 + c f(8) = a*8^2 + b*8 + c

f ( 8 ) = 1 8 8 2 + 1 4 8 + 1 \Rightarrow f(8) = \frac{1}{8}*8^2 + \frac{1}{4}*8 + 1

f ( 8 ) = 8 + 2 + 1 = 11 \Rightarrow f(8) = 8+2+1 = \boxed{11}

W de Weijer
Jul 26, 2013

f ( x ) f(x) is a quadratic polynomial, so we can write it as:

f ( x ) = a x 2 + b x + c f(x) = ax^2 + bx + c .

Subsituting this in the equations gives:

  1. 0 a + 0 b + c = 1 c = 1 0a + 0b + c = 1 \Leftrightarrow c = 1

  2. 16 a + 4 b + 1 4 a 2 b 1 = 12 a + 2 b = 2 16a + 4b + 1 - 4a - 2b - 1 = 12a + 2b = 2

  3. 36 a + 6 b + 1 16 a 4 b 1 = 20 a + 2 b = 3 36a + 6b + 1 - 16a - 4b - 1 = 20a + 2b = 3

Solving 2 and 3 for a a and b b :

2 b = 2 12 a 20 a 2 + 12 a = 32 a 2 = 3 a = 1 8 2b = 2 - 12a \Rightarrow 20a - 2 + 12a = 32a - 2 = 3 \Leftrightarrow a = \frac{1}{8}

12 8 2 = 2 b b = 1 4 \frac{12}{8} - 2 = 2b \Leftrightarrow b = \frac{1}{4}

Now we that know all the coefficients, we can simply calculate f ( 8 ) f(8) :

1 8 × 8 2 + 1 4 × 8 + 1 = 11 \frac{1}{8} \times 8^2 + \frac{1}{4} \times 8 + 1 = 11

If f ( x ) f(x) is a quadratic polynomial, then it must be in the form a x 2 + b x + c ax^2+bx+c . Plugging in x = 0 x=0 yields f ( 0 ) = a 0 2 + b 0 + c = 0 + 0 + c = c f(0)=a \cdot 0^2+b \cdot 0+c=0+0+c=c . Since we know that f ( 0 ) = 1 f(0)=1 and f ( 0 ) = c f(0)=c , then f ( 0 ) = c = 1 c = 1 f(0)=c=1 \Rightarrow c=1 . Therefore, we know that f ( x ) f(x) is in the form a x 2 + b x + 1 ax^2+bx+1 . When plugging in f ( 2 ) f(2) , f ( 4 ) f(4) , and f ( 6 ) f(6) , we have f ( 2 ) = 4 a + 2 b + 1 f(2)=4a+2b+1 , f ( 4 ) = 16 a + 4 b + 1 f(4)=16a+4b+1 , and f ( 6 ) = 36 a + 6 b + 1 f(6)=36a+6b+1 . Therefore, f ( 4 ) f ( 2 ) = 16 a + 4 b + 1 ( 4 a + 2 b + 1 ) = 16 a + 4 b + 1 4 a 2 b 1 = ( 16 a 4 a ) + ( 4 b 2 b ) + ( 1 1 ) = 12 a + 2 b f(4)-f(2)=16a+4b+1-(4a+2b+1)=16a+4b+1-4a-2b-1=(16a-4a)+(4b-2b)+(1-1)=12a+2b . This is equal to 2, by the given condition f ( 4 ) f ( 2 ) = 2 f(4)-f(2)=2 . Also, f ( 6 ) f ( 4 ) = ( 36 a + 6 b + 1 ) ( 16 a + 4 b + 1 ) = ( 36 a 16 a ) + ( 6 b 4 b ) + ( 1 1 ) = 20 a + 2 b f(6)-f(4)=(36a+6b+1)-(16a+4b+1)=(36a-16a)+(6b-4b)+(1-1)=20a+2b . This is equal to 3, by the given condition. Here, we have a system of equations: 12 a + 2 b = 2 12a+2b=2 and 20 a + 2 b = 3 20a+2b=3 . Subtracting the second equation from the first, we have 8 a = 1 a = 1 8 8a=1 \Rightarrow a=\frac{1}{8} . Plugging this in back to the first equation from the system of equations, we have 12 ( 1 8 ) + 2 b = 2 3 2 + 2 b = 2 2 b = 1 2 b = 1 4 . 12(\frac{1}{8})+2b=2 \Rightarrow \frac{3}{2}+2b=2 \Rightarrow 2b=\frac{1}{2} \Rightarrow b=\frac{1}{4}. . Now, we have our complete polynomial: f ( x ) = 1 8 x 2 + 1 4 x + 1 f(x)=\frac{1}{8} x^2+\frac{1}{4}x+1 , and f ( 8 ) = 1 8 8 2 + 1 4 8 + 1 = 1 8 64 + 1 4 8 + 1 = 8 + 2 + 1 = 11 f(8)=\frac{1}{8} \cdot 8^2+\frac{1}{4} \cdot 8+1=\frac{1}{8} \cdot 64+\frac{1}{4} \cdot 8+1=8+2+1=\boxed{11}

Fatik Redy Hanif
Jul 24, 2013

Construct f ( x ) = a x 2 + b x + c f(x)=ax^2+bx+c , we get f ( 0 ) = c = 1 f(0)=c=1

Substituting x x with 2 , 4 2,4 and 6 6 , we get

f ( 4 ) f ( 2 ) = 12 a + 2 b = 2 ( 1 ) f(4)-f(2)=12a+2b=2 \ldots (1)

f ( 6 ) f ( 4 ) = 20 a + 2 b = 3 ( 2 ) f(6)-f(4)=20a+2b=3 \ldots (2)

from ( 1 ) (1) and ( 2 ) (2) , we can easily get a = 1 8 a=\frac {1}{8} and b = 1 4 b=\frac {1}{4} . Thus f ( 8 ) = 1 8 8 2 + 1 4 8 + 1 = 11 f(8)=\frac {1}{8} \cdot 8^2+\frac {1}{4} \cdot 8+1=11

Linh Tran
Jul 24, 2013

Denote the polynomial P ( x ) P(x) by a x 2 + b x + c ax^2+bx+c , we have:

P ( 0 ) = c P ( 2 ) = 4 a + 2 b + c P ( 4 ) = 16 a + 4 b + c P ( 6 ) = 36 a + 6 b + c \begin{matrix} P(0) = c \\ P(2) = 4a+2b+c \\ P(4) = 16a+4b+c \\ P(6) = 36a+6b+c \end{matrix}

According to the problem, we have:

{ c = 1 12 a + 2 b = 2 20 a + 2 b = 3 \left\{ \begin{matrix} c=1 \\ 12a+2b = 2 \\ 20a+2b = 3 \end{matrix} \right.

Hence we obtain:

a = 1 8 b = 1 4 c = 1 \begin{matrix} a=\frac18 \\ b=\frac 14 \\ c=1 \end{matrix}

So P ( x ) = x 2 8 + x 4 + 1 P(x) = \frac {x^2}{8} + \frac {x}{4} +1 , therefore P ( 8 ) = 11 P(8) = 11 .

Raymond Lin
Jul 22, 2013

Since f ( x ) f(x) is a quadratic, it can be written in the form a x 2 + b x + c ax^2+bx+c .

f ( 0 ) = 1 f(0)=1 , so a ( 0 ) 2 + b ( 0 ) + c = 1 a(0)^2+b(0)+c=1 , or c = 1 c=1 . Therefore, f ( x ) = a x 2 + b x + 1 f(x)=ax^2+bx+1 .

f ( 4 ) f ( 2 ) = 2 f(4)-f(2)=2 , so 16 a + 4 b + 1 ( 4 a + 2 b + 1 ) = 2 16a+4b+1-(4a+2b+1)=2 , or 12 a + 2 b = 2 12a+2b=2 .

f ( 6 ) f ( 4 ) = 3 f(6)-f(4)=3 , so 36 a + 6 b + 1 ( 16 a + 4 b + 1 ) = 3 36a+6b+1-(16a+4b+1)=3 , or 20 a + 2 b = 3 20a+2b=3 .

Therefore, f ( 8 ) = 64 a + 8 b + 1 = 2 ( ( 12 a + 2 b ) + ( 20 a + 2 b ) ) + 1 f(8)=64a+8b+1=2((12a+2b)+(20a+2b))+1 .

Substituting in the values for 12 a + 2 b 12a+2b and 20 a + 2 b 20a+2b which we got earlier, we get that f ( 8 ) = 2 ( 3 ) + 2 ( 2 ) + 1 = 11 f(8)=2(3)+2(2)+1=\fbox{11} .

Jordi Bosch
Jul 22, 2013

We have a quadratic polynimial represented by ax^2 + bx + c If f(0) equals to 1 this means that a 0^2 + b 0 + c = 1 And so, we get that c = 1 Once we know that we can easily find the other values using a sistem of equations. We know from f(4) - f(2) = 0 that 16a + 4b + 1 - (4a + 2b + 1) = 2, which simplified is 6a + b = 1 From f(6) - f(4) we get this other equation: 20a + 2b = 3. * Solving this two linear equations we get that * a = 1/8 and b = 1/4. * Now we can assume that our polynomial has this estructure: * 1/8x^2 + 1/4x + 1= 0 And now we can calculate f(8) which equals to 11 . SOLVED

Irfan Amirafi
Jul 22, 2013

We know that quadratic equation have maximum degree of 2.

So we write the general equation for quadratic equation is :

ax^2 + bx + c = 0

We have f(0) = 1, so 0 + 0 + c = 1

c = 1 ... (1)

f(4)-f(2) = 2. so

16a + 4b + 1 - 4a - 2b - 1 = 2

12a + 2b = 2 ... (2)

f(6)-f(4) = 3, so

36a + 6b + 1 -16a -4b -1 = 3

20a + 2b = 3 ... (3)

Solving equation (2) and (3) we have :

a = 1/8 and b = 1/4

Substitute that to f(8) = 64a + 8b + 1 = 64(1/8) + 8(1/4) + 1 = 11

Ahaan Rungta
Jul 21, 2013

First, note that, since f ( x ) f(x) is quadratic and f ( 0 ) = 1 f(0) = 1 , we can say that it is of the form f ( x ) = a x 2 + b x + 1 f(x) = ax^2 + bx + 1 . Now, we are given two conditions to solve for a a and b b . From the second equation, we have ( 16 a + 4 b + 1 ) ( 4 a + 2 b + 1 ) = 2 12 a + 2 b = 2. (16a + 4b + 1) - (4a + 2b + 1) = 2 \implies 12a + 2b = 2. From the second equation, we have ( 36 a + 6 b + 1 ) ( 16 a + 4 b + 1 ) = 3 20 a + 2 b = 3. (36a + 6b + 1) - (16a + 4b + 1) = 3 \implies 20a + 2b = 3. Note that we want f ( 8 ) = 64 a + 8 b + 1 = 4 ( 16 a + 2 b ) + 1. f(8) = 64a + 8b + 1 = 4 \cdot (16a + 2b) + 1. Referring to our previous equations, we have f ( 8 ) = 4 ( 12 a + 2 b + 4 a ) + 1 = 4 ( 2 + 4 a ) + 1 = 9 + 16 a . f(8) = 4 \cdot (12a + 2b + 4a) + 1 = 4 \cdot (2 + 4a) + 1 = 9 + 16a. Now, subtracting our original two equations, we have 8 a = 1 8a = 1 , so 16 a = 2 16a = 2 and f ( 8 ) = 9 + 16 a = 9 + 2 = 11 f(8) = 9 + 16a = 9 + 2 = \boxed {11} .

Pratyush Sahoo
Aug 25, 2015

let the quadratic equation be in the form ax^2 + bx + c

putting the values and soling by taking f(0,2,3) respectively we get a = 1/8 b= 1/4 c = 1

now putting the value as f(8)= 64a+8b+c = 64 X 1/8 + 8 X 1/4 + 1 = 11

Jc Ng
Jul 28, 2013

Let f(x) = a x 2 x^{2} +bx+c

f(0) = c = 1

f(4)-f(2) = 16a+4b+c-4a-2b-c = 2

12a+2b = 2 -------------------(1)

6a+b = 1

f(6)-f(4) = 36a+6b+c-16a-4b-c = 3

20a+2b = 3 -------------------(2)

(2)-(1): 8a = 1

a = 1 8 \frac{1}{8}

b = 1- 6 8 \frac{6}{8} = 1 4 \frac{1}{4}

c = 1

f(x) = 1 8 \frac{1}{8} x 2 x^{2} + 1 4 \frac{1}{4} x+1

Therefore, when x=8, f(8) = 11

A.J Pradana
Jul 28, 2013

f(x) = ax" + bx + c f(0) = 1 c = 1

f(4) - f(2) = 2 16a + 4b + 1 - (4a + 2b + 1) = 2 12a + 2b = 2 6a + b = 1 ... (1)

f(6) - f(4) = 3 36a + 6b + 1 - (16a + 4b + 1) = 3 20a + 2b = 3 10a + b = 3/2 ... (2)

eliminate (1) & (2) 4a = 1/2 a = 1/8 .. (3)

6a + b = 1 [subs (3)] 6/8 + b = 1 b = 2/8 = 1/4

So, f(8) = (1/8) . 8" + (1/4) . 8 + 1 = 8 + 2 +1 = 11

Since it is the question of comparing Coefficients. So we have: f(0) = 1, f(4) - f(2) = 2 and f(6) - f(4) = 3. therefore, f(4) - f(2) = f(6) - f(4) = f(2) since, [f(4) - f(2)] + [f(4) - f(2)] + [f(6) - f(4)] + [f(6) - f(4)] + [f(0)] f(8) = 2 + 2 + 3 + 3 + 1 = 11. ans

Now , the question will arise in your mind that why would we add f(0) the answer is f(0) is given in the question and it is the one of the coefficient that's why we add f(0).

Shivang Jindal
Jul 27, 2013

Just see , if quadratic equation is , a x 2 + b x + c ax^2+bx+c then since f ( 0 ) = 1 c = 1 f(0)=1 \implies c=1
The other two relation give 6 a + b = 1 , 20 a + 2 b = 3 6a+b=1 , 20a+2b=3 which on solving gives a = 1 / 8 , b = 1 / 4 a=1/8 , b = 1/4 thus the result :)

Tran Trung Nguyen
Jul 27, 2013

Let f(x)=ax^2+bx+c => f(0) =1 ; change x into f(x) with f(2);f(4) and f(6), we have a=1/8; b=1/4 => f(8)=8.(1/8)^2 + 8.(1/4) + 1 =11

Akash Pradhan
Jul 26, 2013

Let f ( x ) = A x 2 + B x + C f(x) = Ax^{2} + Bx + C Then f ( 0 ) = 1 C = 1 f(0) = 1 \implies C = 1

f ( x ) = A x 2 + B x + 1 \therefore f(x) = Ax^{2} + Bx + 1

Now, f ( 2 x ) f ( 2 x 2 ) = 8 A x 4 A + 2 B f(2x) - f(2x-2) = 8Ax - 4A + 2B

putting x = 2 x = 2 and x = 3 x = 3 successively; we get :

f ( 4 ) f ( 2 ) = 12 A + 2 B = 2 f(4) - f(2) = 12A + 2B = 2

f ( 6 ) f ( 4 ) = 20 A + 2 B = 3 f(6) - f(4) = 20A + 2B = 3

On solving the systems of equations we get A = 1 8 A = \frac {1}{8} and B = 1 4 B = \frac {1}{4}

So, f ( x ) = x 2 8 + x 4 + 1 f ( 8 ) = 11 f(x) = \frac {x^{2}}{8} + \frac {x}{4} + 1 \therefore f(8) = 11

Ivan Sekovanić
Jul 25, 2013

Each quadratic polynomial can be written in the form of a x 2 + b x + c ax^2+bx+c , where a , b a,b and c c are the real coefficients of the polynomial.

Having that in mind, we may use the first equation to find the value of c c in our polynomial

f ( 0 ) = a 0 + b 0 + c a 0 + b 0 + c = 1 c = 1 f(0)=a\cdot0+b\cdot0+c \Rightarrow a\cdot0+b\cdot0+c=1 \Rightarrow c=1 .

Having the value of c c in mind, we may now create a system using the following 2 2 equations, since f ( 4 ) = 16 a + 4 b + 1 , f ( 2 ) = 4 a + 2 b + 1 f(4)=16a+4b+1, f(2)=4a+2b+1 and f ( 6 ) = 36 a + 6 b + 1 f(6)=36a+6b+1 . Therefore,

{ 12 a + 2 b = 2 20 a + 2 b = 3 \left\{ \begin{array}{l l} 12a+2b=2& \quad \\ 20a+2b=3 & \quad \end{array} \right.

from which we can easily find that a = 1 8 a=\frac{1}{8} and b = 1 4 b=\frac{1}{4} .

The polynomial we are looking for is 1 8 x 2 + 1 4 x + 1 \frac{1}{8}x^2+\frac{1}{4}x+1 . With that in mind, we can easily see that f ( 8 ) = 1 8 64 + 1 4 8 + 1 f ( 8 ) = 8 + 2 + 1 f ( 8 ) = 11 f(8)=\frac{1}{8}64+\frac{1}{4}8+1 \Rightarrow f(8)=8+2+1 \Rightarrow f(8)=11 , which is the correct answer to the problem.

Andrew Edwards
Jul 25, 2013

f ( x ) = a x 2 + b x + c f(x) = ax^2 + bx + c

f ( 0 ) = 1 c = 1 f(0) = 1 \implies c = 1

f ( x ) = a x 2 + b x + 1 f(x) = ax^2 + bx + 1

f ( 4 ) f ( 2 ) = 16 a + 4 b 4 a 2 b = 2 f(4)-f(2) = 16a + 4b - 4a - 2b = 2

f ( 4 ) f ( 2 ) = 36 a + 6 b 16 a 4 b = 3 f(4)-f(2) = 36a + 6b - 16a - 4b = 3

\­(f(4)-f(2) = 12a + 2b = 2)

\­(f(4)-f(2) = 20a + 2b = 3 )

Therefore,

\­(8a= 1 \implies a = \frac{1}{8} )

Hence,

12 8 + 2 b = 2 b = 1 4 \frac{12}{8} + 2b = 2 \implies b = \frac{1}{4}

Finally,

\­(f(x) = \frac{1}{8}x^2 + \frac{1}{4}x + 1 )

So f ( 8 ) = 8 + 2 + 1 = 11 f(8)=8 + 2 + 1=11

John cyril Claur
Jul 25, 2013

Let f(x)=ax^2 + bx +c since f(0)=1, we can conclude that c=1, now we have, f(x)=ax^2 + bx +1 since f(4)-f(2)=2 16a+4b+1-(4a+2b+1)=2 6a+b=1................equation 1

also, f(6)-f(4)=3 36a+6b+1-(16a+4b+1)=3 20a+2b=3..............................equation 2

solve a and b using equation 1 and 2, we get a=1/8 b=1/4

finally, f(x)=(1/8)x^2 + (1/4)x +1

and f(8)=(1/8) 8^2 + (1/4) 8 +1 f(8)=11

Atif Fasih
Jul 24, 2013

For quadratic polynomial use ax^2+bx+c=0. Find out f(0),f(2),f(4),f(6)... and put them in given equations to find a,b &c.. finally get your f(x) and put x=8.!!

Shubham Kumar
Jul 24, 2013

Let f(x) = ax^{2} + bx + c

f(0) = 1, c = 1.

f(2) = 4a + 2b + 1 , f(4) = 16a + 4b +1 , f(6) = 36a + 6b + 1.

f(4) - f(2) = 2

12a + 2b = 2 ...........(i)

f(6) - f(4) = 3

20a + 2b = 3 ............(ii)

On solving (i) & (ii), a = 1/8, b = 1/4.

Therefore, f(x) = (1/8)x^{2} + (1/4)x + 1, and f(8) = 8 + 2 +1 = 11 (Ans.)

Christopher Boo
Jul 24, 2013

Let f ( x ) = a x 2 + b x + c f(x)=ax^2+bx+c

f ( 0 ) = 1 f(0)=1

a ( 0 ) 2 + b ( 0 ) + c = 1 a(0)^2+b(0)+c=1

c = 1 c=1

f ( 4 ) f ( 2 ) = 2 f(4)-f(2)=2

[ a ( 4 ) 2 + b ( 4 ) + c ] [ a ( 2 ) 2 + b ( 2 ) + c ] = 2 [a(4)^2+b(4)+c]-[a(2)^2+b(2)+c]=2

12 a + 2 b = 2 ( 1 ) 12a+2b=2-----(1)

f ( 6 ) f ( 4 ) = 3 f(6)-f(4)=3

[ a ( 6 ) 2 + b ( 6 ) + c ] [ a ( 4 ) 2 + b ( 4 ) + c ] = 3 [a(6)^2+b(6)+c]-[a(4)^2+b(4)+c]=3

20 a + 2 b = 3 ( 2 ) 20a+2b=3-----(2)

From ( 1 ) (1) and ( 2 ) (2) , we have

a = 1 8 a=\frac{1}{8}

b = 1 4 b=\frac{1}{4}

Now, we arrange them and get

f ( x ) = 1 8 x 2 + 1 4 x + 1 f(x)=\frac{1}{8}x^2+\frac{1}{4}x+1

So, f ( 8 ) = 11 f(8)=11

f(x) = ax^2+bx+c

f(0) = 1, c = 1

f(4)-f(2) = 2, 16a+4b+1-(4a+2b+1) = 2, 12a+2b=2

f(6)-f(4) = 3, 36a+6b+1-(16a+4b+1) = 3, 20a+2b=3

a= 1/8, b=1/4, f(8)=11

Adrian Duong
Jul 23, 2013

By method of differences, 1 = f ( 2 ) f ( 0 ) = f ( 2 ) 1 1 = f(2) - f(0) = f(2) - 1 so f ( 2 ) = 2 , f ( 4 ) = 6 , f ( 6 ) = 7 f(2) = 2, f(4) = 6, f(6) = 7 and f ( 8 ) = f ( 6 ) + 4 = 7 + 4 = 11 f(8) = f(6) + 4 = 7 + 4 = 11 .

Lucas Rodrigues
Jul 23, 2013

f(x) = ax^2+bx+c, já que f(x) é um polinômio quadrático.Assim, f(x) = ax^2 + bx + 1. Utilizando as equações de recorrência e somando-as chegamos em a=1/8 e b=1/4. Assim, f(x) = (1/8).x^2 + (1/4)x + 1. Para f(8) achamos 11.

Yash Kumar Gupta
Jul 23, 2013

Let Quadratic Equation be f(x)= ax^2 + bx + c Its given f(0)=1 so from here we get c=1 now f(4) - f(2) = 2 we have => 6a+b=1 and f(6) - f(4) = 3 => 20a+2b=3 solving for 'a' and 'b' we have => a=1/8 and b=1/4 so f(8) = (8) (8) (1/8) + (8)*(1/4) +1 = 11 :)

Gabriel Merces
Jul 23, 2013

The polynomial is ax^2 + bx + 1. Now substitute the other two equations, which give you, 12a + 2b=2; 20a + 2b= 3. Hence 8a=1, 4b=1; hence f(8)=64a + 8b +1= 8 + 2 + 1 = 11.

Harsa Mitra
Jul 23, 2013

Solve the 3 equations to get the value of coefficients i.e. a,b and c. c=1, 12a+2b+c=2 20a+2b+c=3

Which gives:- a = 1/8, b = 1/4, c = 1

So now for f(8)=64 1/8 + 8 1/4 +1=11

Debjit Mandal
Jul 23, 2013

Let, the quadratic polynomial is f(x) = a x 2 x^2 + bx + c
f(0) = c = 1 [given]
So, f(4) - f(2) = 4 2 4^2 a + 4b + 1 - 2 2 2^2 a - 2b - 1 = 2 [given] ⇒12a + 2b = 2...........(1)
and, f(6) - f(4) = 6 2 6^2 a + 6b + 1 - 4 2 4^2 a - 4b - 1 = 3 [given]
⇒20a + 2b = 3...........(2)
By equating '2b' from both equations (1) and (2), we shall get, ⇒2 - 12a = 3 - 20a
⇒8a = 1 ⇒a = \frac {1}{8} \
By putting the value of 'a' in equation (1), we shall get,
b = \frac {1}{4}
So, f(8) = \frac {1}{8} * 8 2 8^2 + \frac {1}{4} *8 + 1 = 8 + 2 + 1 = 11[ANSWER]








Quadratic polynomial has a form: f ( x ) = a x 2 + b x + c f(x)=ax^2+bx+c

So by replacing values we get:

a × 0 2 + b × 0 + c = 1 a \times 0^2 + b \times 0 +c = 1

4 2 a + 4 b + c ( 2 2 a + 2 b + c ) 4^2a+4b+c-\left( 2^2a+2b+c \right)

6 2 a + 6 b + c ( 4 2 a + 4 b + c ) 6^2a+6b+c-\left( 4^2a+4b+c \right)

By solving these equations we get that

a = 1 8 a=\frac{1}{8}

b = 1 4 b=\frac{1}{4}

c = 1 c=1

Now we know that our function has a form: f ( x ) = 1 8 x 2 + 1 4 x + 1 f(x) = \frac{1}{8}x^2+\frac{1}{4}x+1

By replacing 8 in the x we get 11

Daniel Griza
Jul 23, 2013

Let f ( x ) = a x 2 + b x + c f(x)=ax^2+bx+c . Since f(0)=1 => c=1; f ( 4 ) f ( 2 ) = a ( 4 2 2 2 ) + b ( 4 2 ) = 2 f(4)-f(2)=a(4^2-2^2)+ b(4-2)=2 ; and f ( 6 ) f ( 4 ) = a ( 6 2 4 2 ) + b ( 6 4 ) = 3 f(6)-f(4)= a(6^2-4^2)+b(6-4)=3 ;

We get 12 a + 2 b = 2 12a+2b=2 and 20 a + 2 b = 3 20a+2b=3 .

Solving the equations we have a = 1 / 8 a=1/8 and b = 1 / 4 b=1/4 .

So f ( 8 ) = a 8 2 + b 8 + c = 64 / 8 + 8 / 4 + 1 = 8 + 2 + 1 = 11 f(8)=a*8^2+b*8+c=64/8+8/4+1=8+2+1=11

Raghav Chaudhary
Jul 22, 2013

Let us suppose that the quadratic polynomial is ax^2+bx+c. From the first condition, we have that c =1. Now , if we take the second condition, we get the following equation: 12a + 2b = 2 [EQUATION (A)] and if we take the third condition, we get 20a + 2b = 3 [EQUATION (B)] Our job is to find f(8), i.e. 64a + 8b +1. Now let us add equations (A) and (B). We get
32a + 4b = 5. Interestingly, we get 64a + 8b +1 = 2(32a +4b) +1 = 2(5) + 1 = 11. Hence f(8) = 11. (I hope it is clear how we got these linear equations, (by substituting for x).)

Sean Elliott
Jul 22, 2013

Let the quadratic be a x 2 + b x + c ax^2+bx+c Thus we get a system of equations:

0 a + 0 b + c = 1 0a+0b+c=1

( 16 a 2 + 4 b + c ) ( 4 a 2 + 2 b + c ) = 2 (16a^2+4b+c)-(4a^2+2b+c)=2

( 36 a 2 + 6 b + c ) ( 16 a 2 + 4 b + c ) = 3 (36a^2+6b+c)-(16a^2+4b+c)=3

Linear combination gives us a = 1 8 , b = 1 4 , c = 1 a=\frac{1}{8}, b=\frac{1}{4}, c=1

Thus the answer is 64 8 + 8 4 + 1 = 11 \frac{64}{8}+\frac{8}{4}+1=\boxed{11}

Pedro Ferreira
Jul 22, 2013

F(0) = AX^2+BX+C=1 => C=1 F(4)-F(2)=2 => 16A+4B+1-(4A+2B+1)=2 12A+2B=2 => 6A+B=1 F(6)-F(4)=3 => 36A+6B+1-(16A+4B+1)=3 => 20A + 2B = 3 => 10A+B=3/2

Solving the system: 10A+B=3/2 6A+B=1 (-1) 4A=1/2 => A = 1/8 and B = 1/4

Hence, 1/8 * 64+1/4 * 8+1=8+2+1=11

f(x)=ax²+bx+c

f(0)=c=1

f(4)-f(2)=12a+2b=2 ...(i)

f(6)-f(4)=20a+2b=3 ...(ii)

f(8)=64a+8b+1 ...(iii)

mas, fazendo: 2.(i)+2.(ii), obtemos: 64a+8b=10

logo, substituindo em (iii), temos f(8)=10+1=11

because quadratic polynomial, so d2 is constant, from f(4)-f(2)=2 and f(6)-f(4)=3 we know that d2=1 then f(2)-f(0)=1, f(2)=2, f(4)=3, f(6)=7 and f(8)-f(6)=4, f(8)=11

Otávio Sales
Jul 22, 2013

c=1 (f(0)=1)

f(4) - f(2) = 2, then 16a + 4b + 1 - 4a - 2b - 1 = 2 12 a + 2b = 2 6a + b = 1 b= 1 - 6a

f(8) - f(4) = 3, then 36a + 6b + 1 - 16a - 4b - 1 = 3 20a + 2b = 3

Then 20a + 2 - 12a = 3 8a = 3-2 8a = 1 a = 1/8

b= 1 -6/8 b= 2/8 = 1/4

1/8 x 64 + 1/4 x 8 + 1 8 + 2 + 1 11

Mircea Digulescu
Jul 22, 2013

f(x) = a x^2 + b x + c => f(0) = c, so c = 1 By computing f in terms of a and b for the remaining two relationships, we get a system of two linear equations in two unknowns: 12 a + 2 b = 2 20 a + 2 b = 3. It follows a = 1/8, b = 1/4.

Barometer Nongbri
Jul 22, 2013

Let f(x) = ax^2 + bx + c then f(o) = 1 gives c = 1 f(4) - f(2) = 2 gives 12a + 2b = 2 ........(1) f(6) - f(4) = 3 gives 20a +2b = 3 ....... (2)

solving for a and b from (1) and (2) we shal get a= 1/8 and b= 2/8

Put x = 8, a = 1/8 , b= 2/8 , c = 1 in the first expression we get f(8) = 11

Advitiya Brijesh
Jul 22, 2013

Assume , The Quadratic to be f ( x ) = a x 2 + b x + c f(x)=ax^2+bx+c . Now, Using the First Constraint we find that c = 1 c=1 . Similarly using other two equations we found a = 1 8 a=\frac{1}{8} , b = 1 4 b=\frac{1}{4} .

So, Our Quadratic becomes f ( x ) = 1 8 x 2 + 1 4 x + 1 f(x)=\frac{1}{8}x^2+\frac{1}{4}x+1 .

Then, f ( 8 ) = 11 f(8)=11 .

Jan J.
Jul 22, 2013

Let f ( x ) = a x 2 + b x + c f(x) = ax^2 + bx + c , first equality implies c = 1 c = 1 and two other equalities give us system $$ 12a + 2b = 2$$ $$20a + 2b = 3$$ Solving this gives us ( a , b , c ) = ( 1 8 , 1 4 , 1 ) (a,b,c) = \left(\frac{1}{8},\frac{1}{4},1\right) So $$f(8) = \frac{8^2}{8} + \frac{8}{4} + 1 = \boxed{11}$$

Marcus Seto
Jul 22, 2013

Since f ( x ) f(x) is a quadratic polynomial, let f ( x ) = a x 2 + b x + c f(x) = ax^2 + bx + c . Since f ( 0 ) = 1 f(0) = 1 , substituting x = 0 x = 0 into f ( x ) f(x) , c = 1 c = 1 . Hence f ( x ) f(x) can be rewritten as f ( x ) = a x 2 + b x + 1 f(x) = ax^2 + bx + 1 .

Now, f ( 4 ) f ( 2 ) = 2 f(4) - f(2) = 2 . By substitution, we get 16 a + 4 b + 1 ( 4 a + 2 b + 1 ) = 2 12 a + 2 b = 2 16a + 4b + 1 - (4a + 2b + 1) = 2 \Rightarrow 12a + 2b = 2 .

Repeat this for f ( 6 ) f ( 4 ) = 3 f(6) - f(4) = 3 . In this case we get 36 a + 6 b + 1 ( 16 a + 4 b + 1 ) = 3 20 a + 2 b = 3 36a + 6b + 1 - (16a + 4b + 1) = 3 \Rightarrow 20a + 2b = 3 .

We now have a system of linear equations that we can solve for: { 12 a + 2 b = 2 ( 1 ) 20 a + 2 b = 3 ( 2 ) \begin{cases} 12a + 2b = 2 - (1)\\ 20a + 2b = 3 - (2) \end{cases}

( 2 ) ( 1 ) : 8 a = 1 a = 1 8 (2) - (1): 8a = 1 \Rightarrow a = \frac{1}{8} . Substituting a = 1 8 a = \frac{1}{8} into ( 1 ) (1) , 12 8 + 2 b = 2 b = 1 4 \frac{12}{8} + 2b = 2 \Rightarrow b = \frac{1}{4} . Hence the polynomial f ( x ) = 1 8 x 2 + 1 4 x + 1 f(x) = \frac{1}{8}x^2 + \frac{1}{4}x + 1 .

Finding f ( 8 ) f(8) is now just a matter of substituting x = 8 x = 8 into f ( x ) f(x) . f ( 8 ) = 1 8 ( 8 2 ) + 1 4 ( 8 ) + 1 = 11 f(8) = \frac{1}{8}(8^2) + \frac{1}{4}(8) + 1 = \fbox{11} .

R Kumar
Jul 22, 2013

let equation be ax^2+bx+c=0 f(0)=1=>c=1 f(4)−f(2)=2=> 6a+b=1 f(6)−f(4)=3=>10a+b=3/2 on solving a=1/8 b=1/4 and c=1 then quadratic equation is (1/8)x^2+(1/4)x+1=0 and f(8)=11

Danny He
Jul 22, 2013

f ( x ) = a x 2 + b x + c f\left(x\right) = ax^2+bx+c

f ( 0 ) = c = 1 f\left(0\right) = c = 1

f ( 4 ) f ( 2 ) = ( 16 a + 4 b + 1 ) ( 4 a + 2 b + 1 ) f\left(4\right) - f\left(2\right) = \left(16a+4b+1\right) - \left(4a+2b+1\right)

= 12 a + 2 b = 2 = 12a + 2b = 2

b = 1 6 a b = 1-6a

f ( 6 ) f ( 4 ) = ( 36 a + 6 b + 1 ) ( 16 a + 4 b + 1 ) = 3 f\left(6\right) - f\left(4\right) = \left(36a+6b+1\right) - \left(16a+4b+1\right) = 3

20 a + 2 b = 3 20a + 2b = 3

20 a + 2 12 a = 3 20a + 2 -12a = 3

a = 1 8 a = \frac{1}{8}

1 ( 1 8 ) 6 = 2 8 = b 1 - \left(\frac{1}{8}\right)*6 = \frac{2}{8} = b

f ( x ) = 1 8 x 2 + 2 8 x + 1 f\left(x\right) = \frac{1}{8}x^2 + \frac{2}{8}x + 1

f ( 8 ) = 1 8 8 2 + 2 8 8 + 1 f\left(8\right) = \frac{1}{8}*8^2+ \frac{2}{8}*8+ 1

= 8 + 2 + 1 = 11 = 8+2+1 = 11

Iris Romero
Jul 22, 2013

f(0)=1, f(4)−f(2)=2, f(6)−f(4)=3

f(x)=Ax2 +Bx +C

f(0)=1

Ax2+Bx+C=1

0+0+C=1

C=1

f(4)−f(2)=2

16A+4B-(4A+2B)=2

12A+ 2B =2 *Eq. 1

f(6)−f(4)=3

36A+6B-(16A+4B)=3

20A+ 2B =3 *Eq. 2

Eq1&2

20A+2B=3

12A+2B=2

A=1/8

B=2/8 or 1/4

f(x)=1/8x2+1/4x+1

f(8)=8+2+1

f(8)=11

Zi Song Yeoh
Jul 22, 2013

Let f ( x ) = a x 2 + b x + c f(x) = ax^{2} + bx + c . These equations implies

1. c = 1 1. c = 1

2.12 a + 2 b = 2 2. 12a + 2b = 2

3.20 a + 2 b = 3 3. 20a + 2b = 3

Solving these equations gives a = 1 8 , b = 1 4 , c = 1 a = \frac{1}{8}, b = \frac{1}{4}, c = 1 .

So, f ( 8 ) = 11 f(8) = 11 .

Armin Namavari
Jul 22, 2013

Our equation will be in the form f ( x ) = a x 2 + b x + c f(x) = ax^2 + bx + c , since f ( 0 ) = 1 f(0) = 1 , c = 1 c = 1 Now with the last two of the three statements about f, we can set up a system of equations 12 a + 2 b = 2 12a + 2b = 2 20 a + 2 b = 3 20a + 2b = 3 solving this we get a = 1 8 , b = 1 4 a = \dfrac{1}{8}, b = \dfrac{1}{4} , now we have f ( x ) = 1 8 x 2 + 1 4 x + 1 f(x) = \dfrac{1}{8}x^2 + \dfrac{1}{4}x + 1 f ( 8 ) = 8 + 2 + 1 = 11 f(8) = 8 + 2 + 1 = 11

Chen S
Jul 22, 2013

Since it's a quadratic function, we put it in the form f(x) = a x 2 ax^{2} + bx + c.

And f(0) = 1, so c=1.

Next, f(4) = 16a + 4b + 1

f(2) = 4a + 2b + 1

And their difference would be 12a + 2b which equals 2

Next, f(6) = 36a + 6b + 1

and f(4) = 16a + 4b + 1

and their difference is 20a + 2b which equals 3.

So since we now have that 12a + 2b = 2 and 20a + 2b = 3 , we then solve these two equations to get that a = 1/8 and b = 1/4

So our final equation is thus f(x) = 1 / 8 x 2 1/8x^{2} + 1/4x + 1 , so to get f(8), what's left to do is to substitute 8 for x in the equation. So we get 8+2+1 = 11.

Gil Deon Basa
Jul 22, 2013

Let f(x)=ax^2+bx+c if f(0)=1, then 1=a(0)^2+b(0)+c; 1=c if f(4)-f(2)=2, then 2=a(4)^2+b(4)+c-(a(2)^2+b(2)+c); 1=6a+b if f(6)-f(4)=3, then 3=a(6)^2+b(6)+c-(a(4)^2+b(4)+c); 3=20a+2b

We now have c=1; 6a+b=1 and 20a+2b=20. Solving for a and b, we have a=1/8 and b=1/4. Therefore, f(x)=(1/8)x^2+(1/4)x+1 and f(8)=(1/8)(8)^2+(1/4)(8)+1=8+2+1=11

Rajat Garg
Jul 22, 2013

let f(x)=ax^2+bx+c Since f(0)=1 ,c=1 and f(x)=ax^2+bx+1 f(4)-f(2)=12a+2b+1=2 equation-1 f(6)-f(4)=20a+2b+1=3 equation-2 solving 1&2 gives a=1/8 &b=1/4 f(8)=64a+8b+1=8+2+1=11

Mayank Kaushik
Jul 21, 2013

f(0)= 1

a x 2 + b x + c = 0 ax^{2} + bx + c = 0 so c = 1

f(4) - f(2) = 2

16 a + 4 b + 1 4 a 2 b 1 = 2 16a + 4b + 1 - 4a - 2b - 1 = 2

12a + 2b = 1 .......... (1)

f(6) - f(4) = 3

36 a + 6 b + 1 16 a 4 b 1 = 3 36a + 6b + 1 - 16a - 4b - 1 = 3

20a + 2b = 3 ............(2)

on subtracting .......(1) from .......(2)

8a = 1

a = (\frac{1}{8})

so b = 1- 6 8 \frac{6}{8} = 1 4 \frac{1}{4}

f(8) = 1 8 8 2 + 1 4 8 + 1 \frac{1}{8} *8^{2} + \frac{1}{4}*8 + 1 = 8 + 2 + 1 = 11

Duc Minh Phan
Jul 21, 2013

Suppose that f ( x ) = a x 2 + b x + c f(x)=ax^2+bx+c , we have c = f ( 0 ) = 1 c=f(0)=1 , f ( 4 ) f ( 2 ) = 12 a + 2 b f(4)-f(2)=12a+2b , f ( 6 ) f ( 4 ) = 20 a + 2 b f(6)-f(4)=20a+2b . Then we can find a = 1 8 a=\frac{1}{8} and b = 1 4 b=\frac{1}{4} . Therefore f ( 8 ) = 8 2 8 + 8 4 + 1 = 11 f(8) = \frac{8^2}{8}+\frac{8}{4}+1=11 .

Anup Raj
Jul 21, 2013

Let assume quadratic polynomial to be f ( x ) = a . x 2 + b . x + c f(x) = a.x^2 + b.x + c .

Now, f ( 0 ) = 1 f(0) = 1 a . 0 + b . 0 + c = 1 \Rightarrow a.0 + b.0 + c = 1 . Therefore, c = 1 \boxed {c = 1} .

It is given that,

f ( 4 ) f ( 2 ) = 2 16. a + 4. b + c ( 4. a + 2. b + c ) = 2 f(4) - f(2) = 2 \Rightarrow 16.a + 4.b + c - ( 4.a + 2.b + c ) = 2 ( 1 ) \ldots \ldots(1)

f ( 6 ) f ( 4 ) = 3 36. a + 6. b + c ( 16. a + 4. b + c ) = 3 f(6) - f(4) = 3 \Rightarrow 36.a + 6.b + c - ( 16.a + 4.b + c ) = 3 ( 2 ) \ldots \ldots(2)

From (1) and (2),

12. a + 2. b = 2 12.a + 2.b = 2 and 20. a + 2. b = 3 20.a + 2.b = 3 . Solving these equations simultaneously gives a = 1 8 , b = 1 4 \boxed {a = \frac {1}{8}} , \boxed {b = \frac {1}{4}} .

Hence, the polynomial f ( x ) = x 2 8 + x 4 + 1 f(x) = \frac {x^2}{8} + \frac {x}{4} + 1 . Puting x = 8 x = 8 ,

f ( x ) = 11 \boxed {f(x) = 11} .

Solve for the three equations: a ( 0 ) 2 + b ( 0 ) + c = 1 a(0)^2 + b(0) + c = 1 ( a ( 4 ) 2 + b ( 4 ) + c ) ( a ( 2 ) 2 + b ( 2 ) + c ) = 2 (a(4)^2 +b(4) + c) - (a(2)^2 +b(2) + c) = 2 ( a ( 6 ) 2 + b ( 6 ) + c ) ( a ( 4 ) 2 + b ( 4 ) + c ) = 3 (a(6)^2 +b(6) + c) - (a(4)^2 +b(4) + c) = 3

Get, a = 1/8; b = 2/8; c= 1

Solution: ( a ( 8 ) 2 + b ( 8 ) + c ) = 11 (a(8)^2 +b(8) + c) = 11

Caio Pelicioni
Jul 21, 2013

Because the polynomial is quadratic, I am assuming it has the form f ( x ) = a x 2 + b x + c f(x) = ax^{2} + bx + c .

The first equation gives us c = 1 c = 1 .

The second and the third equation, when using the values given and the value of c c reached, show us a system:

6 a + b = 1 6a + b = 1

20 a + 2 b = 3 20a + 2b = 3

Solving the system, we get to a = 1 8 a = \frac{1}{8} and b = 1 4 b = \frac{1}{4}

So, our polynomial is, actually, f ( x ) = x 2 8 + x 4 + 1 f(x) = \frac{x^{2}}{8} + \frac{x}{4} + 1

Therefore, f ( 8 ) = 64 8 + 8 4 + 1 f(8) = \frac{64}{8} + \frac{8}{4} + 1

f ( 8 ) = 8 + 2 + 1 f(8) = 8 + 2 + 1

f ( 8 ) = 11 f(8) = 11

Let our polynomial be f(x) = ax^2 + bx + c.

Now, f(0) = c => c = 1

f(4) - f(2) = 2 => 16a + 4b + c - 4a - 2b - c = 2 => 12a + 2b = 2 [equation 1]

f(6) - f(4) = 3 => 20a + 2b = 3 [equation 2]

Solving equations [1] and [2] we have a= ( \frac{1}{8} ), b = ( \frac{1}{8} ).

So f(x) = ( \frac{1}{8}\ ) x^2 + ( \frac{1}{4}\ ) x + 1

So f(8) = 11

Takeda Shigenori
Jul 21, 2013

Let f ( x ) = a x 2 + b x + c f(x)=ax^2+bx+c , f ( 0 ) = c = 1 f(0)=c=1 , f ( 4 ) f ( 2 ) = 12 a + 2 b = 2 f(4)-f(2)=12a+2b=2 , f ( 6 ) f ( 4 ) = 20 a + 2 b = 3 f(6)-f(4)=20a+2b=3 . Solving the simultaneous equations, a = 1 8 , b = 1 4 , c = 1 a=\frac{1}{8}, b=\frac{1}{4}, c=1 . Therefore, f ( x ) = 1 8 x 2 + 1 4 x + 1 f(x)=\frac{1}{8}x^2+\frac{1}{4}x+1 , f ( 8 ) = 8 + 2 + 1 = 11 f(8)=8+2+1=11 .

Tim Vermeulen
Jul 21, 2013

Define g ( x ) = f ( 2 x ) g(x) = f(2x) . We know that

f ( 0 ) = g ( 0 ) = 1 , f ( 4 ) f ( 2 ) = g ( 2 ) g ( 1 ) = D 1 ( 1 ) = 2 , f ( 6 ) f ( 4 ) = g ( 3 ) g ( 2 ) = D 1 ( 2 ) = 3. \begin{aligned} f(0) = g(0) &= 1,\\ f(4) - f(2) = g(2) - g(1) = D_1(1) &= 2,\\ f(6) - f(4) = g(3) - g(2) = D_1(2) &= 3. \end{aligned}

It follows that

D 2 ( 1 ) = D 1 ( 2 ) D 1 ( 1 ) = 2 1 = 1. D_2(1) = D_1(2) - D_1(1) = 2 - 1 = 1.

We are given that f ( x ) f(x) is a quadratic polynomial, so g ( x ) g(x) is one too. Therefore, D 2 ( x ) D_2(x) is constant for all x x , so D 2 ( 0 ) = D 2 ( 1 ) = D 2 ( 2 ) = 1 D_2(0) = D_2(1) = D_2(2) = 1 . It follows that

D 1 ( 1 ) D 1 ( 0 ) = D 2 ( 0 ) = 1 D 1 ( 0 ) = D 1 ( 1 ) D 2 ( 0 ) = 2 1 = 1 , D_1(1) - D_1(0) = D_2(0) = 1 \implies D_1(0) = D_1(1) - D_2(0) = 2 - 1 = 1,

and

D 1 ( 3 ) D 1 ( 2 ) = D 2 ( 2 ) D 1 ( 3 ) = D 2 ( 2 ) + D 1 ( 2 ) = 1 + 3 = 4. D_1(3) - D_1(2) = D_2(2) \implies D_1(3) = D_2(2) + D_1(2) = 1 + 3 = 4.

So, this is what we know about D 1 ( x ) D_1(x) :

D 1 ( 0 ) = 1 D 1 ( 1 ) = 2 D 2 ( 2 ) = 3 D 2 ( 3 ) = 4 D_1(0) = 1\\ D_1(1) = 2\\ D_2(2) = 3\\ D_2(3) = 4

Now, let's move on to g ( x ) g(x) :

g ( 1 ) g ( 0 ) = D 1 ( 0 ) g ( 1 ) = D 1 ( 0 ) + g ( 0 ) = 1 + 1 = 2 g ( 2 ) g ( 1 ) = D 1 ( 1 ) g ( 2 ) = D 1 ( 1 ) + g ( 1 ) = 2 + 2 = 4 g ( 3 ) g ( 2 ) = D 1 ( 2 ) g ( 3 ) = D 1 ( 2 ) + g ( 2 ) = 3 + 4 = 7 g ( 4 ) g ( 3 ) = D 1 ( 3 ) g ( 4 ) = D 1 ( 3 ) + g ( 3 ) = 4 + 7 = 11 \begin{aligned} g(1) - g(0) = D_1(0) &\implies g(1) = D_1(0) + g(0) = 1 + 1 = 2\\ g(2) - g(1) = D_1(1) &\implies g(2) = D_1(1) + g(1) = 2 + 2 = 4\\ g(3) - g(2) = D_1(2) &\implies g(3) = D_1(2) + g(2) = 3 + 4 = 7\\ g(4) - g(3) = D_1(3) &\implies g(4) = D_1(3) + g(3) = 4 + 7 = 11 \end{aligned}

So, f ( 8 ) = f ( 2 × 4 ) = g ( 4 ) = 11 f(8) = f(2 \times 4) = g(4) = \boxed{11} .

This was what i was searching for.. Method of Differences.. Big Thumbs up buddy..

Krishna Jha - 7 years, 10 months ago
Ricardo Alencar
Jul 21, 2013

Knowing that f ( x ) = a x 2 + b x + c f(x) = ax^2 + bx + c we obtain from eq. ( 1 ) (1) : $$ c = 1 $$ From equation ( 2 ) (2) : $$ 12a + 2b = 2 $$ And finally from equation ( 3 ) (3) : $$ 20a + 2b = 3 $$ Adding both equations, we'll get: $$ 32a + 4b = 5 \Rightarrow 64a + 8b = 10 $$ $$ \Rightarrow 64a + 8b + c = 10 + c \Rightarrow f(8) = 10 + 1 = 11 $$

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...