cos80°cos20°+sin100°sin380°=?
问题描述：
cos80°cos20°+sin100°sin380°=?
答
sin100=sin(180-100)=sin80
sin380=sin(360+20)=sin20
cos80°cos20°+sin100°sin380°
=cos80cos20+sin80sini20
=cos(80-20)
cos60
=1/2
公式:cos(a-b)=cosacosb+sinasinb