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Calculus Level 3

x x is a real number that satisfies the equation 2 x = x 2 . 2^x = x^2.

How many such x x 's are there?

0 1 2 3 4

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1 solution

Henry U
Nov 29, 2018

Obviously, x = 2 x=2 is a solution, but also x = 4 x=4 since 2 4 = 4 2 2^4 = 4^2 .

2 x > x 2 2^x>x^2 for x > 4 x>4 ,so there are no other solutions.

For x < 2 x<2 , there is one additional negative solution. 2 0 = 1 > 0 = 0 2 2^0=1>0=0^2 but then 2 1 = 1 2 < 1 = ( 1 ) 2 2^{-1}=\frac 12<1=(-1)^2 . By the Intermediate Value Theorem , there must be one solution in between and since x 2 x^2 is strictly decreasing for x < 0 x<0 , this is the only negative solution

Therefore, there are a total of 3 solutions.

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