A calculus problem by Steven Jim

Calculus Level 4

Find the minimum value of A = a a A=a^a for all a > 0 a>0 . If min ( A ) = x x \min (A)=x^x , find min ( A ) + x \min (A)+x .


The answer is 1.06008.

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1 solution

Chew-Seong Cheong
Aug 16, 2017

A = a a A = a^a , d A d a = ( ln a + 1 ) a a \implies \dfrac {dA}{da} = (\ln a + 1) a^a . When d A d a = 0 \dfrac {dA}{da} = 0 , ln a + 1 = 0 \implies \ln a + 1 = 0 and a = 1 e a = \dfrac 1e . Note that d 2 A d a 2 = a a 1 + ( ln a + 1 ) 2 a a \dfrac {d^2A}{da^2} = a^{a-1} + (\ln a+1)^2a^a , d 2 A d a 2 a = 1 e > 0 \implies \dfrac {d^2A}{da^2} \bigg|_{a=\frac 1e} > 0 , min ( A ) = ( 1 e ) 1 e \implies \min (A) = \left(\frac 1e\right)^\frac 1e and min ( A ) + x = ( 1 e ) 1 e + 1 e 1.060 \min (A) + x = \left(\frac 1e\right)^\frac 1e + \frac 1e \approx \boxed{1.060} .

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