Upon scanning the answer choices, we might recognize that answer choice E, f(x) = 2^x, involves a variable exponent and that the given information that f(a+b) = f (a)f(b) looks A LOT like the Product Law: (k^a)(k^b) = k^(a+b)
So, let's check E first.

E. f(x) = 2^x
If f(x) = 2^x, then f(a) = 2^a, f(b) = 2^b and f(a+b) = 2^(a+b)

We get: f(a)f(b) = (2^a)(2^b)
= 2^(a+b) [apply Product Law]
= f(a+b)
PERFECT!!!

Answer: E

Cheers,
Brent
_________________

Brent Hanneson – GMATPrepNow.com

Sign up for our free Question of the Day emails

Another approach is to test each function to see whether f(a+b) = f(a)f(b)
For example, let's see what happens if a = 1 and b = 1
So, with each function, is it true that f(1 + 1) = f(1)f(1)?
In other words, is it true that f(2) = f(1)f(1)?

A. f(x) = x + 1
Is it true that f(2) = f(1)f(1)?
Plug values into the function to get: 2 + 1 = (1 + 1)(1 + 1)
Simplify: 3 = 4
No good.
ELIMINATE A

B. f(x) = x² + 1
Is it true that f(2) = f(1)f(1)?
Plug values into the function to get: 2² + 1 = (1² + 1)(1² + 1)
Simplify: 4 + 1 = (2)(2)
Simplify: 5 = 4
No good.
ELIMINATE B

C. f(x) = √x
Is it true that f(2) = f(1)f(1)?
Plug values into the function to get: √2 = (√1)(√1)
Simplify: √2 = (1)(1)
No good.
ELIMINATE C

D. f(x) = 1/x
Is it true that f(2) = f(1)f(1)?
Plug values into the function to get: 1/2 = (1/1)(1/1)
Simplify: 1/2 = (1)(1)
No good.
ELIMINATE D

By the process of elimination, the correct answer is E

Cheers,
Brent
_________________

Brent Hanneson – GMATPrepNow.com

Sign up for our free Question of the Day emails