已知0已知0已知0sina=3/5 cosa=4/5sin(a+b)=-3/5cosb=cos(a+b-a)=cos(a+b)cosa+sin(a+b)sina =-4/5*4/5+(-3/5)*3/5=-1
已知0sina=3/5 cosa=4/5sin(a+b)=-3/5cosb=cos(a+b-a)=cos(a+b)cosa+sin(a+b)sina =-4/5*4/5+(-3/5)*3/5=-1