sin 3,5a*sin 2,5a-cos 3,5a*cos 2,5a+cos (4π+ а)=

Sin3,5α* Sin2,5α - Cos3,5α * Cos2,5α + Cos(4π + α) =

= - (Cos3,5α * Cos2,5α - Sin3,5α * Sin2,5α) + Cosα =

= - Cos(3,5α + 2,5α ) + Cosα = Cosα - Cos6α