1)cos2a=cos^2a-sin^2a

sin^2a+cos^2a=1

sin^2a=1-16/25, т.к п/2<a<п

sin^2a=9/25

cos2a=16/25-9/25=7/25

tga=3/5*5/4=3/4

sina=3/5

2) cos135=sqrt2/2

 sin8p/3=sqrt3/2

tg7p/3=2

 cos^2p/8-sin^2p/8=cos2p/8=вопрос