a: \(=\cos^2a\)
b: \(=\sin a\cdot\left(1-\cos^2a\right)=\sin^3a\)
c: \(=\left(\sin^2a+\cos^2a\right)^2=1\)
\(a,=cos^2a\)
\(b,=sin.a\cdot\left(1-cos^2a\right)=sin^3a\)
\(c,=\left(sin^2a+cos^2a\right)^2=1\)
\(d,=sin^2.20^o-sin^240^o-sin^250^o...+cos^260^o=cos^290^o\)
d)
\(=sin^220^o-sin^240^o-sin^250^o+sin^270^o+cos^230^o+cos^260^o=cos^290^o\)