Question

Помогите решить тождество

Answer 1

$frac{(1-cos alpha )*(1+cos alpha )}{sin2 alpha } *ctg alpha = frac{(1-cos^2 alpha )}{2*sin alpha * cos alpha } * frac{cos alpha }{sin alpha } = \ \ = frac{sin^2 alpha }{2*sin alpha * cos alpha } * frac{cos alpha }{sin alpha } = frac{1}{2}$