lingfeng2108

cos75cos165=cos(30+45)*(-cos15)=-cos(30+45)sin(30+45)=-(cos30cos45-sin30sin45)*(sin30cos45+cos30sin45)=-0.25
基本公式
cosx=-cos(180-x)
sin(x+y)=sinxcosy+cosxsiny
cos(x+y)=cosxcosy-sinxsiny