Two Inputs for a Quadratic

Algebra Level 1

Suppose p ( x ) = a x 2 + b x + c p(x) = ax^2 + bx + c is a quadratic polynomial satisfying p ( 7 ) p ( 3 ) = 214. p(7)-p(3)= 214. Determine the value of p ( 9 ) p ( 1 ) p(9)-p(1) .


The answer is 428.

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20 solutions

William Cui
Nov 17, 2013

Let's calculate the values of p ( 7 ) p(7) and p ( 3 ) p(3) :

p ( 7 ) = 7 2 a + 7 b + c = 49 a + 7 b + c p(7)=7^2a+7b+c = 49a + 7b+c

p ( 3 ) = 3 2 a + 3 b + c = 9 a + 3 b + c p(3)=3^2a+3b+c=9a+3b+c

Subtracting, we have p ( 7 ) p ( 3 ) = 49 a + 7 b + c 9 a 3 b c = 40 a + 4 b p(7) - p(3) = 49a+7b+c-9a-3b-c = 40a + 4 b

Try subtracting p ( 9 ) p ( 1 ) p(9) - p(1) too: p ( 9 ) p ( 1 ) = 81 a + 9 b + c a b c = 80 a + 8 b p(9) - p(1) = 81a+9b+c-a-b-c = 80a + 8 b

This is getting us nowhere, right? Look again at the two values. Notice that 2 ( 40 a + 4 b ) = 80 a + 8 b 2(40a+4b)=80a+8b , so the value of p ( 9 ) p ( 1 ) p(9)-p(1) is double the value of p ( 7 ) p ( 3 ) p(7) - p(3) .

Substititing, we have p ( 9 ) p ( 1 ) = 2 214 = 428 p(9) - p(1) = 2 \cdot 214 = \boxed{428}

hey look its william

Matthew Feng - 7 years, 6 months ago

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hey it's mah wife from arml 2013

William Cui - 7 years, 6 months ago

this whole problem may as well have been made without c i do not know why you even mention it in your proof.

pranav kalkunte - 7 years, 6 months ago

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I mention it because it may not be obvious to some why the equations are true; it is less confusing.

William Cui - 7 years, 6 months ago
Adam Zaim
Nov 17, 2013

I ignored this entirely:

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

p ( 7 ) p ( 3 ) = 214 p(7) - p(3) = 214 p ( 7 ) p ( 3 ) = p ( 4 ) p(7) - p(3) = p(4) Therefore...

p ( 4 ) = 214 p(4) = 214

p ( 9 ) p ( 1 ) = p ( 8 ) p(9) - p(1) = p(8)

p ( 4 ) × 2 = p ( 8 ) p(4) \times 2 = p(8)

Therefore...

p ( 8 ) = 214 × 2 = 428 p(8) = 214 \times 2 = 428

So the answer is 428 .

Moderator note:

As pointed out by Chee, this solution is wrong.

Let's put your solution aside, now, can you prove that p ( x ) p ( y ) = p ( x y ) p(x)-p(y)=p(x-y) such that both p ( x ) p(x) and p ( y ) p(y) are polynomials? None of them! You have to know that p ( x ) p(x) is a function, just like the f ( x ) f(x) in the previous problem sets, and not a variable. If p ( x ) p(x) is a variable, then it should be written as p x px not p ( x ) p(x) .

Chee Choy Chong - 7 years, 6 months ago

Wow... you guys...

Mine seems less complicated. XD

Adam Zaim - 7 years, 6 months ago

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As pointed out by Chee, your solution is incorrect. p ( x ) p(x) is a function. p p is not a constant.

Calvin Lin Staff - 7 years, 6 months ago
Chee Choy Chong
Nov 17, 2013

From the question, we can say

p ( 7 ) p ( 3 ) p(7) - p(3) = = 214 214

7 2 a + 7 b + c ( 3 2 a + 3 b + c ) 7^{2}a + 7b + c - (3^{2}a + 3b +c) = = 214 214

49 a + 7 b + c ( 9 a + 3 b + c ) 49a + 7b + c - (9a + 3b +c) = = 214 214

40 a + 4 b 40a + 4b = = 214 214

Then,

p ( 9 ) p ( 1 ) p(9) - p(1)

= = 9 2 a + 9 b + c ( 1 2 a + 1 b + c ) 9^{2}a + 9b + c - (1^{2}a + 1b + c)

= = 81 a + 9 b + c ( a + b + c ) 81a + 9b + c - (a + b + c)

= = 80 a + 8 b 80a + 8b

= = 2 ( 40 a + 4 b ) 2(40a + 4b)

= = 2 ( 214 ) 2(214)

= = 428 \boxed {428}

sry i still didnt understand

Chua Hao Zhe - 7 years, 6 months ago

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May I know where you get confused? I will try my best to explain.

Chee Choy Chong - 7 years, 6 months ago

You may consider, if you wish, looking at my explanation. That may help you out.

Tamoghna Banerjee - 7 years, 6 months ago
Ayon Pal
Nov 18, 2013

1) We have p ( x ) = a x 2 + b x + c p(x) = ax^2 + bx + c and p ( 7 ) p ( 3 ) = 214 p(7) - p(3) = 214 .

2) The value of p ( 7 ) p(7) , when x = 7 x = 7 in p ( x ) = a x 2 + b x + c p(x) = ax^2 + bx + c :

p ( 7 ) = 7 2 a + 7 b + c p(7) = 7^{2}a + 7b + c

49 a + 7 b + c \implies 49a + 7b + c

3) And the value of p ( 3 ) p(3) , when x = 3 x= 3 in p ( x ) = a x 2 + b x + c p(x) = ax^2 + bx + c :

p ( 3 ) = 3 2 a + 3 b + c p(3) = 3^{2}a + 3b + c

9 a + 3 b + c \implies 9a + 3b + c

4) Work out the value of p ( 7 ) p ( 3 ) p(7) - p(3) :

p ( 7 ) p ( 3 ) = ( 49 a + 7 b + c ) ( 9 a + 3 b + c ) p(7) - p(3) = (49a + 7b + c) - (9a + 3b + c)

40 a + 4 b \implies 40a + 4b

4 ( 10 a + b ) \implies 4(10a + b)

5) Putting the value of p ( 7 ) p ( 3 ) = 214 p(7) - p(3) = 214 :

p ( 7 ) p ( 3 ) = 214 p(7) - p(3) = 214

4 ( 10 a + b ) = 214 \implies 4(10a + b) = 214

10 a + b = 214 4 \implies 10a + b = \frac{214}{4}

6) We get the value of 10 a + b = 214 4 10a + b = \frac{214}{4}

7) p ( 9 ) p ( 1 ) p(9) - p(1) should be [like step 2 or 3] :

p ( 9 ) p ( 1 ) = ( 9 2 a + 9 b + c ) ( 1 2 a + 1. b + c ) p(9) - p(1) = (9^{2}a + 9b + c) - (1^{2}a + 1.b + c)

( 81 a + 9 b + c ) ( a + b + c ) \implies (81a + 9b + c) - (a + b + c)

80 a + 8 b \implies 80a + 8b

8 ( 10 a + b ) \implies 8(10a + b)

8) Putting the value of 10 a + b = 214 4 10a + b = \frac{214}{4} [from step 6] :

p ( 9 ) p ( 1 ) = 8 ( 10 a + b ) p(9) - p(1) = 8(10a + b)

8 × 214 4 \implies 8 \times \frac{214}{4}

2 × 214 428 \implies 2 \times 214 \implies 428

9) We get the value of p ( 9 ) p ( 1 ) = 428 p(9) - p(1) = \boxed{428}

your write men ;)

Marc Justin De Guzman - 7 years, 6 months ago

i just came here to see what other answers were like, but now this website thinks this is my level and does not allow me level up.

pranav kalkunte - 7 years, 6 months ago

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If you work through the Diagnostic (only once), that should update your level.

Calvin Lin Staff - 7 years, 6 months ago
Rohit Rungta
Nov 17, 2013

p(7) - p(3) = 40a + 4b

p(9) - p(1) = 80a + 8b

Since 80a + 8b is double 40a + 4b you just need to double 214 to get 428

Is my way is wrong ? P(7)-p(3) = 214 7p-3p = 214 9p-1p = ? 4p=214 8p=? p=107/2 8(107/2) = 428

Zahir Jamal - 7 years, 6 months ago

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Yes bro.........this is a wrong method..........because "p" is a function in "x" and not a variable................so you can't write.......p(7)=7p......:)

Kshitij Nishan - 7 years, 6 months ago

p(7) = 49a + 7b + c and p(3) = 9a + 3b + c therefore, p(7) - p(3) = 40a + 4b = 214, and then p(9) = 81a + 9b + c and p(1) = a+b+c and p(9) - p(1) = 80a + 8b = 2(40a+4b) = 2(214) = 428

ang galing mo naman sa math , paturo naman po

Russel Araniego - 7 years, 6 months ago
David Kroell
Nov 19, 2013

If you plug 7 and 3 into the equation you get:

[a(49) - b(7) + c] - [a(9) + b(3) + c] = a(40) - b(4)

> If you do the same for P(9) - P(1)

You get: a(80) + b(8)

= 2 [ a(40) + b(4) ] = 2 * 214 = 428

Romeo Gomez
Nov 18, 2013

By the definition we have that p ( 7 ) = 49 a + 7 b + c , p(7)=49a+7b+c, and p ( 3 ) = 9 a + 3 b + c , p(3)=9a+3b+c, so p ( 7 ) p ( 3 ) = 40 a + 4 b = 4 ( 10 a + b ) = 214. p(7)-p(3)=40a+4b=4(10a+b)=214. On the other hand we have p ( 9 ) = 81 a + 9 b + c , p(9)=81a+9b+c, and p ( 1 ) = a + b + c , p(1)=a+b+c, so p ( 9 ) p ( 1 ) = 80 a + 8 b = 8 ( 10 a + b ) = 2 ( 4 ( 10 a + b ) ) . p(9)-p(1)=80a+8b=8(10a+b)=2(4(10a+b)). But 4 ( 10 a + b ) = 214. 4(10a+b)=214. Hence p ( 9 ) p ( 1 ) = 2 ( 214 ) = 428 . p(9)-p(1)=2(214)=\boxed{428}.

Aaron Kelly
Nov 18, 2013

First off I looked for the constant value, the first thing that struck out was that it was the same function both times. I noted this and then inserted the variables into the equation giving; P(7)=49a+7b+c P(3)=9a+3b+c I then inserted these equations into the second equation giving; (49a+7b+c)-(9a+3b+c)=214 Solving gives; 40a+4b=214 I then did the same for the second set of values giving; 80a+8b=x Since it is the same function and we have a linear equation they are directly proportional, e.g if one doubles the other doubles which is what is happening here so; 1/2x=214 x=428

Tamoghna Banerjee
Nov 18, 2013

On solving p(7) - p(3), we arrive at the equation, 40a + 4b = 214. Now, to get to know more about the constants, we need at least two more equations. The present situation seems to be against this idea. So lets keep the deduced equation aside and try to solve p(9) - p(1). Now, on solving this, we end up at 80a + 8b, which is interestingly the double of (40a + 4b). Since we have already found out this value, we multiply it(i.e. 214 ) by 2 to get 428.

Kshitij Nishan
Nov 17, 2013

We have, p(7)-p(3)=(49a+7b+c)-(9a+3b+c)=40a+4b=214 (given)............................Then, p(9)-p(1)=(81a+9b+c)-(a+b+c)=80a+8b=2(40a+4b)=2*214=428.

Sameer Vijaykumar
Nov 17, 2013

Given: Quadratic Equation with variables a, b, c & x [Also p] Equation answering the quadratic given.

To Do: Simplify the equation p(7) - p(3) = 214 p(7-3) = 214 p(4) = 214 p = 214 / 4 p = 53.5 Now, Substitute the value of 'p' in the the next equation.

p(9) - p(1) = ? p(9 -1) = ? p(8) = ?

Substituting the value of 'p'

53.5 * 8 = 428

Moderator note:

As pointed out by Chee, this solution is incorrect. p ( x ) p(x) is a function. p p doesn't have a value of 53.5.

its help me so much . thanks

Munifah Nadzirah - 7 years, 6 months ago

As pointed out by Chee above, your solution is wrong. p ( x ) p(x) is a function, and not the value of p × x p \times x .

Calvin Lin Staff - 7 years, 6 months ago
Benjamin Kan
Jan 17, 2014

We can write p ( 7 ) p ( 3 ) = 214 p(7)-p(3)=214 as 49 a + 7 b + c ( 9 a + 3 b + c ) = 214 49a+7b+c-(9a+3b+c)=214 . Simplifying, we get 40 a + 4 b = 214 40a+4b=214 . Now let's work with p ( 9 ) p ( 1 ) p(9)-p(1) . We can write p ( 9 ) p ( 1 ) p(9)-p(1) as 81 a + 9 b + c ( a + b + c ) 81a+9b+c-(a+b+c) Simplifying, we get 80 a + 8 b 80a+8b . 80 a + 8 b 80a+8b is just 2 ( 40 a + 4 b ) 2\cdot(40a+4b) , and we can substitute 214 into 40 a + 4 b 40a+4b , so 80 a + 8 b = 428 80a+8b=\boxed{428}

nice

Krishna Ramesh - 7 years, 1 month ago
Sharky Kesa
Nov 22, 2013

By inputting 7 and 3 as x x we get:

( 49 a + 7 b + c ) ( 9 a + 3 b + c ) = ( 40 a + 4 b ) (49a + 7b + c) - (9a + 3b + c)=(40a + 4b)

By also inputting 9 and 1 we get:

( 81 a + 9 b + c ) ( a + b + c ) = ( 80 a + 8 b ) (81a + 9b + c) - (a + b + c)=(80a + 8b)

This means that:

p ( 9 ) p ( 1 ) = 2 ( p ( 7 ) p ( 3 ) ) p(9) -p(1)=2*(p(7)-p(3))

Since:

p ( 7 ) p ( 3 ) = 214 p(7)-p(3)=214

p ( 9 ) p ( 1 ) = 428 p(9)-p(1)=428

Nick Smith
Nov 19, 2013

If p(7) - p(3) = 214 then

(49a + 7b + c) - (9a + 3b +c) = 214

40a + 4b = 214

p(9) - p(1) can be defined by the following expression which we'll call x

(81a +9b + c) - (a + b + c) = x

80a + 8b = x

Factoring we get

8(10a + b) = x

Factoring 40a + 4b we get 4(10a + b) = 214

Because 214/(10a + b) = 4 and x/(10a + b) = 8 we know that x must be twice as large as 214 so 428 is our answer

Oussama Jaber
Nov 19, 2013

P(7) = 49a+7b+c
P(3) = 9a+3b+c
P(9) =81a+9b+c
P(1) = a+b+c
P(7)-P(3) = 49a+7b+c-9a-3b-c
= 40a+4b
therefore, 40a+4b=214 ----------------------(1)
P(9)-P(1) = 81a+9b+c-a-b-c
= 80a+8b
therefore, 80a+8b = k ----------------------(2)
Now, we have a system of equation to solve from (1) and (2)
40a+4b=214
80a+8b=k -------> divide by 2 --------> 40a+4b=k/2
So, 214 = k/2 -------------> k = 428












Raakin Kabir
Feb 15, 2016

1 p(7) - p(3) = 214

p(4) = 214

1# p(9) - p(1) = p(8) p(8) is two times the amount of p(4) or 214. 214 * 2 = 428

Andre Yudhistika
Jan 5, 2014

P(7)-P(3)= 49a+7b+c-9a-3b-c=40a+4b=214

P(9)-P(1)=81a+9b+c-a-b-c=80a+8b=2(40a+4b)=2*214=428

p(x) = a x 2 x^2 + bx + c

as we may know in the first case p(7) , x is equal to 7

substitute the value of x, so we obtain these equations

p(7) = 49a+7b+c

p(3) = 9a+3b+c

and then subtract p(3) from p(7),so we obtain that 40a+4b = 214

for the p(9) and p(1)

p(9) = 81a+9b+c

p(1) = a+b+c

then subtract p(1) from p(9) ,we will then obtain 80a+8b

if 40a+4b = 214

80a+8b can be simplified to this form 2(40a+4b)

that is the same to 2(214)

so the value of 80a+80b is 428 \boxed{428}

p(7)-p(3)= 49a+7b+c - (9a+3b+c) = 40a+4b=214 (I)
p(9)-p(1)= 81a+9b+c - (a+b+c) = 80a+ 8b (II)
So, it´s easy to see that if you multiply (I) by 2, it´s (II).
Finally you can find that 80a+8b = 428


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