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Exercise 729Calculus › Single-variable calculus › Integration › Improper integralsL1Added 2 Nov 2026

An integral singular at both endpoints

#improper integrals#substitution#arcsine

Statement

Show that ∫01dxx(1−x)\displaystyle\int_0^1\frac{dx}{\sqrt{x(1-x)}} converges and compute it.

Hint

The integrand blows up at 00 and at 11. Substitute x=sin⁡2θx=\sin^2\theta.

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Solution

Convergence. Near 00, 1x(1−x)∼1x\frac1{\sqrt{x(1-x)}}\sim\frac1{\sqrt x} and ∫0dxx\int_0\frac{dx}{\sqrt x} converges (p=12<1p=\frac12<1). Near 11, the integrand behaves like 11−x\frac1{\sqrt{1-x}}, which also gives a convergent integral. So the integral converges.

Value. Let x=sin⁡2θx=\sin^2\theta with θ∈(0,π2)\theta\in\left(0,\frac\pi2\right). Then dx=2sin⁡θcos⁡θ dθdx=2\sin\theta\cos\theta\,d\theta and x(1−x)=sin⁡θcos⁡θ\sqrt{x(1-x)}=\sin\theta\cos\theta, so
∫01dxx(1−x)=∫0π/22 dθ=π.\int_0^1\frac{dx}{\sqrt{x(1-x)}}=\int_0^{\pi/2}2\,d\theta=\pi.

Remark. Since the substitution is a bijection between the open intervals and the integrand becomes constant, the singularities disappear completely. This integral is the Beta function value B(12,12)=πB\left(\frac12,\frac12\right)=\pi.