Question

Вычислить определенный интеграл

Answer 1

$\int \dfrac{x\ dx}{sin^2x}=\Big[\ u=x\ ,\ du=dx\ ,\ dv=\dfrac{dx}{sin^2x}\ ,\ v=-ctgx\ \Big]=\\\\\\=uv-\int v\, du=-x\cdot ctgx+\int ctgx\, dx=-x\cdot ctgx+\int \dfrac{cosx}{sinx}\, dx=\\\\\\=-x\cdot ctgx+\int \dfrac{d(sinx)}{sinx}=-x\cdot ctgx+ln|sinx|+C\\\\\\\\\int \limits _{\pi /4}^{\pi /3}\dfrac{x\ dx}{sin^2x}=\Big(-x\cdot ctgx+ln|sinx|\Big)\Big|_{\pi /4}^{\pi /3}=$

$=-\dfrac{\pi}{3}\cdot \dfrac{\sqrt3}{3}+ln\dfrac{\sqrt3}{2}-\Big(-\dfrac{\pi}{4}\cdot 1+ln\dfrac{\sqrt2}{2}\Big)=-\dfrac{\pi \sqrt3}{9}+ln\dfrac{\sqrt3}{2}+\dfrac{\pi}{4}-ln\dfrac{\sqrt2}{2}=\\\\\\=\dfrac{(9-4\sqrt3)\, \pi }{36}+ln\sqrt{\dfrac{3}{2}}$