Mathematics

Mathematics

Problem

Evaluate ∫x2log⁡(1−x2) dx\int x^2 \log(1-x^2)\,dx, and hence prove that 11.5+12.7+13.9+⋯=23log⁡2−89\frac{1}{1.5} + \frac{1}{2.7} + \frac{1}{3.9} + \dots = \frac{2}{3} \log 2 - \frac{8}{9}.
Intuitive explanation

This problem hinges on integration by parts combined with a power series expansion. Once you integrate x2log⁡(1−x2)x^2 \log(1-x^2) by parts, you'll get a result involving log⁡(1−x2)\log(1-x^2); then expand that logarithm as a series using log⁡(1−u)=−∑unn\log(1-u) = -\sum \frac{u^n}{n}. The key insight is that evaluating your antiderivative at specific bounds will produce a series whose terms match the pattern in the sum you need to prove.

Intuitive explanation

This problem hinges on integration by parts combined with a power series expansion. Once you integrate x2log⁡(1−x2)x^2 \log(1-x^2) by parts, you'll get a result involving log⁡(1−x2)\log(1-x^2); then expand that logarithm as a series using log⁡(1−u)=−∑unn\log(1-u) = -\sum \frac{u^n}{n}. The key insight is that evaluating your antiderivative at specific bounds will produce a series whose terms match the pattern in the sum you need to prove.

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