(1-sin a)*(1+sin a)=cos^2 B

Question

(1-sin a)*(1+sin a)=cos^2 B

Answer 1

1/sin^2a=1+ctg^2aпоэтому  1/sin^2a -ctg^2a-cos^2b=1+ctg^2a -ctg^2a-cos^2b=1-cos^2b=sin^2b