Question

已知cos=-1/7,cos（a+b）-11/14,求cosB的值

Answer 1

cosa=-1/7,cos(a+b)=-11/14

sina=±√(1-cos²a)=±4/7√3

sin(a+b)=±√[1-cos²(a+b)]=±5/14√5

cosb=cos[(a+b)-b]

=cos(a+b)cosa+sin(a+b)sina

=-11/14*(-1/7)+(±4/7√3)(±5/14√5)

=11/98±10/49√15