sin4θ+cos4θ =(sin^2θ)^2+(cos^2θ)^2+2(sin^2θ)*cos^2θ)-2(sin^2θ)*cos^2θ) =[(sin^2θ)+(cos^2θ)]^2-2(sin^2θ)*cos^2θ) =1-(1/2)*(sin2θ)^2 =1-(1/2)*[(1-cos4θ)/2 =3/4+(1/4)*cos4θ所以cos4θ=1 接...