Упростить выражение: 1) cos (π/4+a) × cos

1) Sin (a)*(1-Cos^2 (a)=Sin^2 (a)*Sin^2 (a)=Sin^4 (a) 2) Sin^4 (a)+cos^4 (a)+2sin^2 (a) (1-sin^2 (a)=Sin^4 (a)+cos^4 (a)+2sin^2 (a) — 2Sin^4 (a)=cos^4 (a)+2sin^2 (a) — Sin^4 (a)=cos^4 (a)+sin^2 (a)*(2-Sin^2 (a)=cos^4 (a)+sin^2 (a)*(1+1-Sin^2 (a)=cos^4 (a)+sin^2 (a)*(1+Cos^2 (a) 3) cos (a)*(Cos^2 (a)+Sin^2 (a)=cos (a) 4) (cos^3 (a) -sin^3 (a)+(cos (a) sin^2 (a) -sin (a)*cos^2 (a)=(cos^3 (a) -sin^3 (a)+Cos (a)*Sin (a)*(Sin^2 (a) -Cos^2 (a)=Cos (a)*Sin (a)*(Cos^2 (a) -Sin^2 (a)+Sin^2 (A) -Cos^2 (a)=Cos (a)*Sin (a)*0=0