Объяснение:1)
1/cos^2(a) – tg^2(a)-sin^2(a)= 1/cos^2(a) – sin^2(a)/cos^2(a))-sin^2(a)=
=[1 – sin^2(a) ]/cos^2(a))-sin^2(a)=cos^2(a)/cos^2(a))-sin^2(a)=
=1-sin^2(a)=cos^2(a)
