1+Cos(pi-a)*sin(a-3pi/2)

1+(-cosa)*sin(a-2π-π/2)=1+(-cosa)*sin(a-π/2)=1-cosa*sin(-(π/2-a))=1-cosa*(-sin(π/2-a))=1+cosa*sin(π/2-a)=1+cosa*cosa=1+cos²a