Помогите найти значение выражения А=(cos20pi/3 -sin17pi/6)*sin 12.5pi:cos(-15pi/4)

cos(20pi/3)=cos(6pi+pi-pi/3)=cos(pi-pi/3)=-cos(pi/3)=-1/2
sin(17pi/6)=(2pi+pi-pi/6)=sin(pi-pi/6)=sin(pi/6)=1/2.
sin(12,5pi)=sin(12pi+pi/2)=sin(pi/2)=V2/2.
cos(-15pi/4)=cos(15pi/4)=cos(4pi-pi/4)=cos(-pi/4)=cos(pi/4)=V2/2.
A=[(-1/2-1/2)*V2/2]/(V2/2)=-1.