a+b属于（pai/2,pai/2*3）,cos(a+b)=-56/65,sinb=12/13
sin(a)=sin(a+b-b)=sin(a+b)cons(b)-cos(a+b)sin(b)=33/65*(-5/13)-(-56/65)*(12/13)=3/5-56/65是怎么来的0 0cos(a+b)^2+sin(a+b)^2=1