- 三角比已知cosA
- 已知cA-cosB=1/2,sinA-sinB=-1/3,求cos(A-B)
- 解:∵cosA-cosB=1/2,sinA-sinB=-1/3,
∴cos^A+cos^B-2cosA×cosB=1/4.....(1)
sin^A+sin^B-2sinA×sinB=1/9.....(2)
(1)+(2)得:cos^A+cos^B+sin^A+sin^B-2(cosA×cosB+sinA×sinB )=1/4+1/9
∴2-2 cos(A-B)=13/36
cos(A-B)=59/72