cos(a-b)cos(b-c)+sin(a-b)sin(b-c)=
问题描述：
cos(a-b)cos(b-c)+sin(a-b)sin(b-c)=
答
cos(a-b)cos(b-c)+sin(a-b)sin(b-c)
=cos[(a-b)-(b-c)]
=cos(a+c-2b)