(1-cos2альфа)/ (1+tg^2альфа)

Question

(1-cos2альфа)/ (1+tg^2альфа)

Answer 1

(c0s^2a+sin^2a-cos^a+sin^2a)/(1/cos^2a)=2sin^2a*cos^2a=1/2(2sinacosa)^2=1/2sin^2 2a