Question

已知sina=3/5,cosb=-5/13,且a、b都是第二象限,求sin(a-b)与cos(a+b)的值

Answer 1

sina=3/5,cosb=-5/13,a、b都在第二象限

cosa=-4/5,sinb=12/13

sin(a-b)

=sinacosb-cosasinb

=3/5*(-5/13)-(-4/5)*12/13

=33/65

cos(a+b)

=cosacosb-sinasinb

=(-4/5)*(-5/13)-3/5*12/13

=-16/65