a(π/2,π) b(-π/2,0)
a-b(π/2,3π/2)
又sin(a-b)=3/5
所以a-b(π/2,π)
cos(a-b)=4/5
sinb=-12/13 ,cosb=5/13
sina=sin(a-b+b)
=sin(a-b)cosb+cos(a-b)sinb
=3/5*5/13-4/5*12/13
=-33/65