已知sin(A-B)cosA-cos(A-B)sinA=3/5,cos2B=?

问题描述:

已知sin(A-B)cosA-cos(A-B)sinA=3/5,cos2B=?

sin(A-B)cosA-cos(A-B)sinA=3/5

sin(A-B-A)=sin(-B)=3/5
sinB=-3/5
所以
cos2B=1-2sin平方B
=1-2×9/25
=1-18/25
=7/25