Выражения (1-sin^2a)/(cos^2a), (1-cos^2a)/(1-sin^2a), (1+tg^2a)*cos^2a-sin^2a, (ctg^2a+1)*sin^2a-cos^2a

(1-sin²a)/(cos²a)=cos²α/cosα=1; 
(1-cos²a)/(1-sin²a)=sin²α/cos²α=tg²; 
(1+tg²a)*cos²a-sin²a=1/(cos²α)*cos²α-sin²α=1-sin²α=cos²α; 
(ctg²a+1)*sin²a-cos²a=1/(sin²α)*sin²α-cos²α=1-cos²α=sin²α.