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Re: For which of the following functions is f(a+b)=f(b)+f(a)  [#permalink]

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enigma123 wrote:
For which of the following functions is f(a+b)=f(b)+f(a) for all positive numbers a and b?

A. f(x)=x^2
B. f(x)=x+1
C. f(x)=√x
D. f(x)=2/x
E. f(x)=-3x

Can someone please tell me how to solve this? As OA is not provided I started by substituing the value of f(x) in the answer choice but got stuck. Can someone please help?

hi

A: (a + b) ^ 2 is not equal to a^ + b ^2, discard
B: a + b + 1 is not equl to b + 1 + a + 1, discard
C: square rootover (a + b) is not equal to rootover a + rootover b , discard
D: 2/ a+b is not equal to 2/a + 2/b
E: -3 (a + b) is equal to -3a -3b, so this is the correct choice

thanks
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GMAT 1: 470 Q36 V19 GMAT 2: 570 Q44 V24 Re: For which of the following functions is f(a+b)=f(b)+f(a)  [#permalink]

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For which of the following functions is f(a+b)=f(b)+f(a) for all positive numbers a and b?

A. f(x)=x^2
B. f(x)=x+1
C. f(x)=√x
D. f(x)=2/x
E. f(x)=-3x

I substituted x in a and b -> f(a+b)= f(a) + f(b) --> f(x+x)= f(x) + f(x) --> f(2x)= 2 f(x)
so I have to find the option that satisfies this equation.
a) f(2x)= 4x^2 vs 2f(x)=2x^2 ( Not equal)
b) f(2x)=2x+1 vs 2f(x)= 2x+2 ( Not equal)
c) f(2x)= sqrt 2*sqrt x vs 2f(x)= 2 * sqrt x (Not equal)
d) f(2x)=1/x vs 2f(x)= 4/x (Not equal)
e) f(2x)=-6x vs 2f(x)= -6x (equal)
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Re: For which of the following functions is f(a+b)=f(b)+f(a)  [#permalink]

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Top Contributor
enigma123 wrote:
For which of the following functions is f(a+b)=f(b)+f(a) for all positive numbers a and b?

A. f(x)=x^2
B. f(x)=x+1
C. f(x)=√x
D. f(x)=2/x
E. f(x)=-3x

One approach is to plug in numbers. Let's let a = 1 and b = 1

So, the question becomes, "Which of the following functions are such that f(1+1) = f(1) + f(1)?"
In other words, for which function does f(2) = f(1) + f(1)?

A) If f(x)=x², does f(2) = f(1) + f(1)?
Plug in to get: 2² = 1² + 1²? (No, doesn't work)
So, it is not the case that f(2) = f(1) + f(1), when f(x)=x²

B) If f(x)=x+1, does f(2) = f(1) + f(1)?
Plug in to get: 2+1 = 1+1 + 1+1? (No, doesn't work)
So, it is not the case that f(2) = f(1) + f(1)
.
.
.
A, B, C and D do not work.
So, at this point, we can conclude that E must be the correct answer.
Let's check E anyway (for "fun")

E) If f(x)=-3x, does f(2) = f(1) + f(1)?
Plugging in 2 and 1 we get: (-3)(2) = (-3)(1) + (-3)(1)
Yes, it works

Cheers,
Brent
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Re: For which of the following functions is f(a+b)=f(b)+f(a)  [#permalink]

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enigma123 wrote:
For which of the following functions is f(a+b)=f(b)+f(a) for all positive numbers a and b?

A. f(x)=x^2
B. f(x)=x+1
C. f(x)=√x
D. f(x)=2/x
E. f(x)=-3x

We need to determine when f(a + b) = f(a) + f(b). Before we evaluate each answer choice it may be easier to use numerical values for a and b. If we let a = 1 and b = 2, our new function looks like:

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

f(3) = f(1) + f(2)

So we must determine when the output of f(3) equals the sum of the outputs of f(1) and f(2).

Let’s now evaluate each answer choice.

A) f(x) = x^2

f(3) = 3^2 = 9

f(1) = 1^2 = 1

f(2) = 2^2 = 4

Since 9 does not equal 1 + 4, choice A is not correct.

B) f(x) = x + 1

f(3) = 3 + 1 = 4

f(1) = 1 + 1 = 2

f(2) = 2 + 1 = 3

Since 4 does not equal 2 + 3, choice B is not correct.

C) f(x) = √x

f(3) = √3

f(1) = √1 = 1

f(2) = √2

Since √3 does not equal 1 + √2, choice C is not correct.

D) f(x) = 2/x

f(3) = 3/2

f(1) = 2/1 = 2

f(2) = 2/2 = 1

Since 3/2 does not equal 2 + 1, choice D is not correct.

Since we have eliminated all the other answer choices, we know the answer is E. However, let’s show as an exercise that answer choice E satisfies the given property for our choice of numbers:

E) f(x) = -3x

f(3) = -3(3) = -9

f(1) = -3(1) = -3

f(2) = -3(2) = -6

Since -9 equals -3 + (-6), choice E is correct.

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Re: For which of the following functions is f(a+b) = f(a) + f(b)  [#permalink]

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_________________ Re: For which of the following functions is f(a+b) = f(a) + f(b)   [#permalink] 07 Oct 2019, 06:00

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