高三 数学 三角函数 请详细解答,谢谢! (11 18:1:30)

sin200 = sin(180+20) = -sin20
cos700 = cos(2*360-20) = cos(-20) = cos20
sin100 = sin(180-80) = sin80
cos500 = cos(360+140) = cos140 = cos(180-40) = -cos40
原式=-sin20cos20 - sin80cos40