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how many integers [#permalink]

29 Sep 2010, 06:13

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for how many integers n is 2^n= n^2 ?
0
1
2
3
>3

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Re: how many integers [#permalink]

29 Sep 2010, 06:21

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vanidhar wrote:

for how many integers n is 2^n= n^2 ?
0
1
2
3
>3

2^n= n^2 is true for 2 integers:
n=2 --> 2^2=2^2=4;
n=4 --> 4^2=2^4=16.

Well, 2^2=2^2=4 is obvious choices, then after trial and error you'll get 4^2=2^4=16 as well. But how do we know that there are no more such numbers? You can notice that when n is more than 4 then 2^n is always more than n^2 so n can not be more than 4. Also n can not be negative as in this case 2^n won't be an integer whereas n^2 will be.

Answer: C.

NOTE: I think it's worth remembering that 4^2=16=2^4, I've seen several GMAT questions on number properties using this (another useful property 8^2=4^3=2^6=64).

Hope it helps.
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Re: how many integers [#permalink]

29 Sep 2010, 08:03

vanidhar wrote:

for how many integers n is 2^n= n^2 ?
0
1
2
3
>3

It helps to know that the function 2^x is more expansive than x^2 for large positive x and converges quickly to 0 for negative x.

So we know we only have to check small values of x. For positive x, it is easy to see this is true for x=2,4 and then the function 2^x explodes

For negative x, 2^-1 is less than -1^2 already so no negative integers can satisfy the equality

Answer is (c) or 2

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Re: how many integers [#permalink]

30 Sep 2010, 19:53

Easy one. +1 for option C. I helps to memorize the powers of 2 till 2^{10}.
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Re: how many integers [#permalink] 30 Sep 2010, 19:53

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