We ain't forgetting the roots!

Calculus Level 4

A = 0 22 x x x 5 4 3 d x A = \int_0^{22} \sqrt{x\sqrt[3]{x\sqrt[4]{x\sqrt[5]{\cdots}}}} dx

Find A A to the nearest integer.

117 116 118 120 119

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1 solution

Chew-Seong Cheong
Jun 25, 2019

A = 0 22 x x x 5 4 3 d x = 0 22 ( x ( x ( x ( ) 1 5 ) 1 4 ) 1 3 ) 1 2 d x = 0 22 x 1 2 + 1 3 ! + 1 4 ! + 1 5 ! + d x = 0 22 x e 2 d x = x e 1 e 1 0 22 = 2 2 e 1 e 1 118 \begin{aligned} A & = \int_0^{22} \sqrt{x\sqrt[3]{x\sqrt[4]{x\sqrt[5]{\cdots}}}} dx \\ & = \int_0^{22} \left(x\left(x\left(x\left(\cdots \right)^\frac 15\right)^\frac 14\right)^\frac 13\right)^\frac 12 dx \\ & = \int_0^{22} x^{\frac 12 + \frac 1{3!}+\frac 1{4!}+\frac 1{5!}+\cdots} dx \\ & = \int_0^{22} x^{e-2} dx \\ & = \frac {x^{e-1}}{e-1} \bigg|_0^{22} = \frac {22^{e-1}}{e-1} \approx \boxed{118} \end{aligned}

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