Simple Improper Integral from OBM Exam

Calculus Level 3

0 π log 10 ( sin ( x ) ) d x = ? \displaystyle{\int_0^π \log_{10}(\sin(x))}\, dx = ?


The answer is -0.945714.

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1 solution

Chew-Seong Cheong
Sep 17, 2018

Let I I be the given integral.

I = 0 π log 10 ( sin x ) d x Since sin x is symmetrical about x = π 2 = 2 0 π 2 log 10 ( sin x ) d x Using a b f ( x ) d x = a b f ( a + b x ) d x = 0 π 2 ( log 10 ( sin x ) + log 10 ( cos x ) ) d x = 0 π 2 log 10 ( sin x cos x ) d x = 0 π 2 log 10 ( sin 2 x 2 ) d x = 0 π 2 log 10 ( sin 2 x ) d x 0 π 2 log 10 2 d x Let u = 2 x d u = 2 d x = 1 2 0 π log 10 ( sin u ) d u x log 10 2 0 π 2 Note that I = 0 π log 10 ( sin x ) d x = 1 2 I π log 10 2 2 Rearranging = π log 10 2 0.946 \begin{aligned} I & = \int_0^\pi \log_{10} (\sin x) \ dx & \small \color{#3D99F6} \text{Since }\sin x \text{ is symmetrical about }x = \frac \pi 2 \\ & = 2 \int_0^\frac \pi 2 \log_{10} (\sin x) \ dx & \small \color{#3D99F6} \text{Using }\int_a^b f(x) \ dx = \int_a^b f(a+b-x) \ dx \\ & = \int_0^\frac \pi 2 \left(\log_{10} (\sin x) + \log_{10} (\cos x) \right) \ dx \\ & = \int_0^\frac \pi 2 \log_{10} (\sin x \cos x) \ dx \\ & = \int_0^\frac \pi 2 \log_{10} \left(\frac {\sin 2x}2 \right) \ dx \\ & = {\color{#3D99F6}\int_0^\frac \pi 2 \log_{10} (\sin 2x) \ dx} - \int_0^\frac \pi 2 \log_{10} 2 \ dx & \small \color{#3D99F6} \text{Let }u = 2x \implies du = 2\ dx \\ & = {\color{#3D99F6}\frac 12 \int_0^\pi \log_{10} (\sin u) \ du} - x \log_{10} 2\ \bigg|_0^\frac \pi 2 & \small \color{#3D99F6} \text{Note that }I = \int_0^\pi \log_{10} (\sin x) \ dx \\ & = {\color{#3D99F6}\frac 12 I} - \frac {\pi \log_{10} 2}2 \ & \small \color{#3D99F6} \text{Rearranging} \\ & = - \pi \log_{10} 2 \approx \boxed{-0.946} \end{aligned}

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