Integration and radicals

Calculus Level 3

0 1 x x x 2017 2017 2017 d x = 1 1 k \large \int_{0}^{1} \sqrt[2017]{x \ \sqrt[2017]{x \ \sqrt[2017]{x\cdots}}} \ dx = 1-\frac{1}{k}

If k k is a positive integer satisfies the equation above, find k k .


The answer is 2017.

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1 solution

Solution as suggested by @Aaghaz Mahajan

Let y = x x x 2017 2017 2017 y = \sqrt[2017]{x\sqrt[2017]{x\sqrt[2017]{x\cdots}}} .

Then, we have:

y = x y 2017 Raising both sides to the power of 2017 y 2017 = x y y 2016 = x y = x 1 2016 \begin{aligned} y & = \sqrt[2017]{xy} & \small \color{#3D99F6} \text{Raising both sides to the power of }2017 \\ y^{2017} & = xy \\ y^{2016} & = x & \\ \implies y & = x^{\frac 1{2016}} \end{aligned}

Now, we have:

I = 0 1 x x x 2017 2017 2017 d x = 0 1 y d x = 0 1 x 1 2016 d x = [ 2016 2017 x 2017 2016 ] 0 1 = 2016 2017 = 1 1 2017 \begin{aligned} I & = \int_0^1 \sqrt[2017]{x\sqrt[2017]{x\sqrt[2017]{x\cdots}}} \ dx \\ & = \int_0^1 y \ dx \\ & = \int_0^1 x^{\frac 1{2016}} dx \\ & = \left[\frac {2016}{2017}x^{\frac {2017}{2016}} \right]_0^1 \\ & = \frac {2016}{2017} \\ & = 1 - \frac 1{2017} \end{aligned}

k = 2017 \implies k = \boxed{2017}

Or, after the third step, simply substitute y=x^(1/2016) and then integrate to get [2016*{x^(2017/2016)}]/(2017)

Aaghaz Mahajan - 3 years, 8 months ago

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Thanks, Much simpler. I will change it.

Chew-Seong Cheong - 3 years, 8 months ago

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The problem with this assumption is that y 2016 = x y^{2016} = x actually has two solutions: y = ± x 1 2016 y = \pm \ x^{\frac{1}{2016}} .

Zach Abueg - 3 years, 8 months ago

No problem. Welcome

Aaghaz Mahajan - 3 years, 8 months ago

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