Easy Peasy Functions!

Algebra Level 2

Let f ( x ) f(x) be a linear function. Given f ( 6 ) f ( 2 ) = 12 f(6)-f(2)=12 find the value of f ( 12 ) f ( 2 ) f(12)-f(2) .


The answer is 30.

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12 solutions

Esrael Santillan
Jul 25, 2014

Since it is linear function, f ( x ) f(x) satisfies f ( c x ) = c f ( x ) f(cx) = cf(x) . Then,

12 = f ( 6 ) f ( 2 ) = f ( 3 ( 2 ) ) f ( 2 ) = 3 f ( 2 ) f ( 2 ) = 2 f ( 2 ) f ( 2 ) = 6 \begin{aligned} 12 &= f(6) - f(2)\\ &= f(3(2)) - f(2)\\ &= 3f(2) - f(2)\\ &= 2f(2)\\ f(2) &= 6 \end{aligned}

Therefore,

f ( 12 ) f ( 2 ) = 6 f ( 2 ) f ( 2 ) = 5 f ( 2 ) = 5 ( 6 ) = 30 \begin{aligned} f(12) - f(2)&= 6f(2) - f(2) \\ &= 5f(2) \\ &= 5(6) \\ &= \boxed{30} \\ \end{aligned}

Strictly speaking a linear function is f ( x ) = c . x + a f(x) = c.x +a so you can't assume that f ( n x ) = n . f ( x ) f(nx) = n.f(x) but when you do the calculations the a cancels out - so the result is the same.

Tony Flury - 5 years, 9 months ago

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a linear function is a function that obeys the two properties f(kx) = kf(x) and f(x + y) = f(x) + f(y)

geoff taylor - 3 years, 4 months ago

Well, linear functions are real functions which map x to ax+C for some real a and C

Assume the real function in this problem is f(x) = ax+C

We're given that f(6)-f(2)=12

This implies that f(6)-f(2)= 6a + C - (2a+C) = 4a = 12

Solving, we get a = 3

Now, look into the other part

f(12)-f(2)=12a+C -(2a+C) = 10a

Substitute, a with 3 and get the answer as 30

I have a question... I got the answer by using f as an unknown variable. I just looked for the value of f and once I found it, I substituted it. Is it wrong to do this?

Roberto Vázquez - 6 years, 10 months ago

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In this case it doesn't affect the result - but in general f(x) isn't the same as f*x. Even as a linear function f ( x ) = m . x + c f(x) = m.x + c and for non-linear functions the definitions could be much more complex.

Tony Flury - 5 years, 9 months ago
Datu Oen
Aug 18, 2014

Since f ( x ) f(x) is a linear function, then we can compute for its slope f ( x 1 ) f ( x 2 ) x 1 x 2 = m \frac{f(x_1) - f(x_2)}{x_1 - x_2} = m .

Recomputing, we can have f ( x 1 ) f ( x 2 ) = m ( x 1 x 2 ) f(x_1) - f(x_2) = m (x_1 - x_2) .

With this, we now have the following:

f ( 6 ) f ( 2 ) = 12 = m ( 6 2 ) f(6) -f(2) = 12 = m (6 - 2) which produces m = 3 m=3 .

Thus, f ( 12 ) f ( 2 ) = m ( 12 2 ) = 3 ( 10 ) = 30 f(12) -f(2) = m (12 - 2) = 3 (10) = 30

Nice method

sandeep Rathod - 6 years, 6 months ago

Very methodical!

Terrell Bombb - 4 years, 8 months ago
Dan Skal
Jul 18, 2014

• Definition of a linear function: f is called linear if it satisfies (i) f(cx) = cf(x) and (ii) f(x + y) = f(x) + f(y) for any vectors x, y and any scalar constant c.

So,

12 = f(6) - f(2) = f(6) + f(-2) = f(6 - 2) = f(4).

Now,

f(12) - f(2) = f(3 * 4) - f((1/2) * 4) = 3f(4) - (1/2)f(4) = 3 * 12 - (1/2) * 12 = 30.

A linear function is always f ( x ) = m . x + c f(x) = m.x + c i.e. always a straight line when graphed. so you can't assume that either of your two identities hold true - they only do when c is zero. In this special case though - when you are talking about the difference between functions - the c values cancel out.

Tony Flury - 5 years, 9 months ago
Kevin Patel
Jul 16, 2014

Let 6x-2x=12 OR 4x=12...thus x=3.
Now,
to find 12x-2x = 12(3)-2(3) = 36-6 = 30

Rakshit Pandey
Jul 16, 2014

Let the function f ( x ) f(x) be represented as A x + B = 0 Ax+B=0 , as it is the general form of linear algebraic functions.
So,
f ( 6 ) = 6 A + B f(6)=6A+B
f ( 2 ) = 2 A + B f(2)=2A+B
f ( 6 ) f ( 2 ) = ( 6 A + B ) ( 2 A + B ) f(6)-f(2)=(6A+B)-(2A+B)
12 = 6 A + B 2 A B 12=6A+B-2A-B
12 = 4 A 12=4A
A = 3 A=3
So, f ( 12 ) f ( 2 ) = ( 12 A + B ) ( 2 A + B ) f(12)-f(2)=(12A+B)-(2A+B)
f ( 12 ) f ( 2 ) = 10 A \Rightarrow f(12)-f(2)= 10A
f ( 12 ) f ( 2 ) = 30 \Rightarrow f(12)-f(2)= 30





Linear function means st. line. Given change in y = 12 change in .x = 6 - 2 = 4.
Slope = 12/4 = 3. Change in x now is 12 - 2 = 10...
.Hence change in y = 3 * 10 = 30.

Mohammad Khaza
Sep 5, 2017

f ( 6 ) f ( 2 ) = 12 f(6)-f(2)=12 ... or,difference of every f(1)= 12 4 = 3 \frac{12}{4}=3

so, f ( 12 ) f ( 2 ) = f ( 3 × 12 ) f ( 3 × 2 ) = 36 6 = 30 f(12)-f(2)=f(3 \times 12)-f(3 \times 2)=36-6=30

Carlos Salgado
Oct 17, 2019

f(x) is lineal so, when it goes from (2) to (6) takes 4 horizontal steps, and when goes from (2) to (12) takes 10. Then in order to get the second amount of vertical steps we must multiply the first amount, 12, by 10/4 = 2.5, then 24 + 6 = 30.

Mattia Conti
Feb 25, 2018

Write f(x) = ax + b since it is a linear equation,

f(6) - f(2) = 12 is equal to write

6a + b - 2a - b = 12 or

4a = 12

a = 3

so the function is f(x) = 3x + b .

Than f(12) - f(2) = 36 + b - 6 - b = 30

Pieter Breughel
Oct 3, 2016

The definition of a slope is m = (y - y1) / (x - x1) or, ((f(x) - f(x1)) / (x - x1))

We can see what the x values are; for f(6) the x-value is 6, and for f(2) the x-value is 2.

Therefore: ((f(6) - f(2)) / (6 - 2)) = m. Given is that f(6) - f(2) = 12. The slope, m, is thus 12 / 4 = 3.

Since it is give that we are dealing with a linear function, we therefore know that ((f(12) - f(2)) / (12 - 2)) = 3, and so (f(12) - f(2)) = 10 * 3 = 30.

Sai Charan Dasari
Oct 24, 2014

given ,f(x) is a linear function.then let it be ax+b. then,f(6)=6x+b,f(2)=2x+b. then f(6)-f(2)=(6x+b)-(2x+b) => 6x+b-2x-b=12 => 4x=12 => x=3 for f(12)-f(2)=12x+b-2x-b=12(3)-2(3)=36-6=30

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