Question

Найдите производную функции

Answer 1

$f(x)=\frac{ctgx}{1+ctg^2x}=\frac{ctgx}{\frac{1}{sin^2x}}=ctgx\cdot sin^2x\\\\f'(x)=-\frac{1}{sin^2x}\cdot sin^2x+ctgx\cdot 2\, sinx\cdot cosx=\\\\=-1+2\cdot \frac{cosx}{sinx}\cdot sinx\cdot cosx=-1+2cos^2x=-(sin^2x+cos^2x)+2cos^2x=\\\\=cos^2x-sin^2x=cos2x$