tg^2ß(1-sinß)(1+sinß)

tg^2(b)=sin^2(b)/cos^2(b)
(1-sinb)×(1+sinb)=1-sin^2(b)=cos^2(b)
Значит (sin^2(b)/cos^2(b))×cos^2(b)=sin^2(b)