Question

请问这个三角函数等式左边是怎么化成等式右边的?

Answer 1

和差化积

追问

非常感谢!

Answer 2

证明：右边=(1/2)(cost-cos3t)
=(1/2)[cos(2t-t)-cos(2t+t)]
=(1/2)[cos2t*cost+sin2t*sint-(cos2t*cost-sin2t*sint)]
=(1/2)(2sin2t*sint)=sint*sin2t=左边
等式得证