题目

已知cosa=1/3,则cos(a+2b)+2sin(a+b)sinb=

提问时间：2021-01-07

答案

cos(a+2b)+2sin(a+b)sinb
=cos[(a+b)+b]+2sin(a+b)sinb
=[cos(a+b)cosb-sin(a+b)sinb]+2sin(a+b)sinb
=cos(a+b)cosb+sin(a+b)sinb
=cos[(a+b)-b]
=cosa
=1/3