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Source: — Problem Solving |

by Anurag@Gurome » Sun Nov 25, 2012 9:06 am
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by Brent@GMATPrepNow » Sun Nov 25, 2012 9:35 am
B166418 wrote:for which of the following functions f is f(x)=f(1-x) for all x

A) f(x)=1-x
B) f(x)=1-x^2
C) f(x)=x^2-(1-x)^2
D) f(x)=x^2(1-x)^2
E) f(x)=x/1-x
Let's try plugging in an easy value for x. How about x = 0.
So, we can reword the question as, For which of the following functions is f(0)=f(1-0)
In other words, we're looking for a function such that f(0) = f(1)

A) f(x)=1-x
f(0)=1-0 = 1
f(1)=1-1 = 0
Since f(0) doesn't equal f(1), eliminate A

B) f(x) = 1 - x^2
f(0) = 1 - 0^2 = 1
f(1) = 1 - 1^2 = 0
Since f(0) doesn't equal f(1), eliminate B

C) f(x) = x^2 - (1-x)^2
f(0) = 0^2 - (1-0)^2 = -1
f(1) = 1^2 - (1-1)^2 = 1
Since f(0) doesn't equal f(1), eliminate C

D) f(x) = x^2(1-x)^2
f(0) = 0^2(1-0)^2 = 0
f(1) = 1^2(1-1)^2 = 0
Since f(0) equals f(1), keep D for now

E) f(x) = x/(1-x)
f(0) = 0/(1-0) = 0
f(1) = 1/(1-1) = undefined
Since f(0) doesn't equal f(1), eliminate E

Since only D satisfies the condition that f(x)=f(1-x) when x=0, the correct answer is D

Cheers,
Brent
Brent Hanneson - Creator of GMATPrepNow.com
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by B166418 » Sun Nov 25, 2012 10:24 am
yeah even i thought by doing this way but it took much time ....

was wondering if some other way of doing this
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by Brent@GMATPrepNow » Sun Nov 25, 2012 10:44 am
B166418 wrote:yeah even i thought by doing this way but it took much time ....

was wondering if some other way of doing this
You can also analyze each function algebraically (as Anurag does here: https://www.beatthegmat.com/gmat-functio ... tml#411296), but that approach also takes time.

It comes down to what you feel more comfortable with: plugging in numbers vs. algebraic manipulation

Cheers,
Brent
Brent Hanneson - Creator of GMATPrepNow.com
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