cos(a-b)=12/13推出sin(a-b)=5/13
sin(a+b)=-3/5推出cos(a+b)=-4/5
sin2a=sin[(a+b)+(a-b)]=sin(a+b)cos(a-b)+cos(a+b)sin(a-b)=(-3/5)*12/13+(-4/5)*5/13=-36/65-20/65=-56/65