Question

(1+ctg² a)*(1-sin² a)

Answer 1

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