Sum that Equals to One

Calculus Level 2

What is the value of x x in the equation below in the nearest thousandths?

n = 2 1 n x = 1 \sum_{n=2}^\infty \frac{1}{n^{x}} = 1


The answer is 1.729.

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1 solution

Kaizen Cyrus
May 31, 2020

n = 2 1 n 1.7 1.05429 > 1 > n = 2 1 n 1.8 0.88223 n = 2 1 n 1.72 1.01592 > 1 > n = 2 1 n 1.73 0.99755 n = 2 1 n 1.728 1.00118 > 1 > n = 2 1 n 1.729 0.99936 n = 2 1 n 1.7286 1.00009 > 1 > n = 2 1 n 1.7287 0.9999 \small \begin{aligned} \sum_{n=2}^\infty \frac{1}{n^{1.7}} ≈ 1.05429 & > 1 > \sum_{n=2}^\infty \frac{1}{n^{1.8}} ≈ 0.88223 \\ \sum_{n=2}^\infty \frac{1}{n^{1.72}} ≈ 1.01592 & > 1 > \sum_{n=2}^\infty \frac{1}{n^{1.73}} ≈ 0.99755 \\ \sum_{n=2}^\infty \frac{1}{n^{1.728}} ≈ 1.00118 & > 1 > \sum_{n=2}^\infty \frac{1}{n^{1.729}} ≈ 0.99936 \\ \sum_{n=2}^\infty \frac{1}{n^{1.7286}} ≈ 1.00009 & > 1 > \sum_{n=2}^\infty \frac{1}{n^{1.7287}} ≈ 0.9999 \end{aligned}

The value of x x is 1.7286 \approx 1.7286 or 1.729 \boxed{1.729} .

Is there a better convincing solution rather than hit and trial?

Mahdi Raza - 1 year ago

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Sadly, this is the only way I did it.

Kaizen Cyrus - 1 year ago

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