Question

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

Answer 1

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