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For how many integers k is k^2 = 2^k ?

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For how many integers k is k^2 = 2^k ? [#permalink] New post 03 Jan 2012, 05:35
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Difficulty:

  35% (medium)

Question Stats:

57% (01:47) correct 43% (00:37) wrong based on 189 sessions
For how many integers k is k^2 = 2^k ?

(A) None
(B) One
(C) Two
(D) Three
(E) More than three
[Reveal] Spoiler: OA

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Re: k^2 = 2^k [#permalink] New post 03 Jan 2012, 07:34
i my case memorization of the square from 1 to30 helped. got C with little doubt in mind.
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Re: k^2 = 2^k [#permalink] New post 03 Jan 2012, 10:25
Looking at it you can know that 2^k has a much greater growth rate than k^2 so based on this and the very quick growth of both functions you should start pluging in numbers with 0 and going up (as negative numbers squared can never equal anything raised to a negative number) you will find that it works for 1 and 2 and that at 4 2^k is much greater than k^2 so you can stop.
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Re: k^2 = 2^k [#permalink] New post 03 Jan 2012, 23:58
Expert's post
metallicafan wrote:
For how many integers k is k^2 = 2^k ?

(A) None
(B) One
(C) Two
(D) Three
(E) More than three

Is there a way to solve it algebraicaly? If we pick numbers, how could we be sure that we have reviewed all the scenarios?


I have discussed a more generic question on this link: try-this-one-700-level-number-properties-103461.html#p805817

Go through it. It should give you multiple ideas.
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Re: k^2 = 2^k [#permalink] New post 04 Jan 2012, 11:58
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Easy, only two ways this can happen 2^2 = 2^2 and 2^4 = 4^2. C
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Re: k^2 = 2^k [#permalink] New post 05 Jan 2012, 05:27
We have to pick numbers,check all multiples of 2 but after 4 none satisfies hence the OA is C.
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Re: k^2 = 2^k [#permalink] New post 04 Jan 2013, 23:17
metallicafan wrote:
For how many integers k is k^2 = 2^k ?

(A) None
(B) One
(C) Two
(D) Three
(E) More than three

Is there a way to solve it algebraicaly? If we pick numbers, how could we be sure that we have reviewed all the scenarios?


Take logs on both sides => 2*logk = k*log2. This helps better visualization of the 2 answers 2, 2^2.
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Re: For how many integers k is k^2 = 2^k ? [#permalink] New post 05 Jan 2013, 19:45
Analytically it is not possible to see that these are the only 2 integer solutions. But you can find out that these are the ONLY possible solutions.
For this, TRY GRAPHING

The best way to see that these are the only solutions is to look at the graph of these two functions:
f(k) = kˆ2 is a parabola with vertex on (0,0)
g(k) = 2ˆk is a exponential curve that has (0,1) as its y intersect

If you trace both graphs you'll see that the tail of the exponential curve does in fact cross the parabola for a value of k < 0 . However you can easily cross out the possibility of this being an integer value of k by testing out k = -1 and k = -2. (ps: Just for curiosity ,this value is a non rational value, aprox. -0.76)

For positive values, there are two intersections (that happen to be integers!). They are, in fact, the trivial k=2 and k=4 solutions most people guessed by testing numbers. It is not hard to see that the first intersection will have to happen, and given how the rate of increase for the exponential curve picks up much faster than the rate of increase for the quadratic function, they will have to cross paths once more. After that, the rate of increase for the quadratic does not catch up to the exponential, making it impossible for a third positive intersection.

Hope it helped!
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Re: For how many integers k is k^2 = 2^k ? [#permalink] New post 11 Aug 2014, 07:30
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Re: For how many integers k is k^2 = 2^k ? [#permalink] New post 13 Aug 2014, 00:27
4^2 = 2^4
2^2 = 2^2

Answer = C
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Re: For how many integers k is k^2 = 2^k ?   [#permalink] 13 Aug 2014, 00:27
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