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=?
答
sin(A-B)cosA-cos(A-B)sinA=3/5
即
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