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Functions

Expert replies
by DCJ » Mon Jun 04, 2012 8:39 am
For which of the following functions is f(a+b)=f(a)+f(b) for all positive numbers a and b?

f(x)=x^2
f(x)=x+1
f(x)=sqrt(x)
f(x)=2/x
f(x)=-3x

What's the best way to solve this without trying each answer choice? Had I looked at choice E first, I would have recognized the use of distributive property and that E was the right answer but instead I started by trying all the answer choices in order.
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Source: — Problem Solving |

by Jim@StratusPrep » Mon Jun 04, 2012 9:06 am
There really is no other way than looking at each answer. The key is to have a grasp of what principles each answer induces so you don't have to do the math for each.

a) eliminates middle term from FOIL
b) add 1 twice
c) same as a
d) common denominator will change numerator
e) your answer
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by Anurag@Gurome » Mon Jun 04, 2012 9:47 am
DCJ wrote:For which of the following functions is f(a+b)=f(a)+f(b) for all positive numbers a and b?
...
What's the best way to solve this without trying each answer choice?
The given relation f(a + b) = f(a) + f(b) is the additive property which holds for most of the linear relations. If you know this you'll check for the linear equations first.

Now, we have only two linear relations f(x) = (x + 1) and f(x) = -3x.
Even without checking each of them, we can see that the former adds an extra 1.

Now, check for f(x) = -3x ---> f(a + b) = -3(a + b) = -3a -3b = f(a) + f(b)
Anurag Mairal, Ph.D., MBA
GMAT Expert, Admissions and Career Guidance
Gurome, Inc.
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