sin(π/4-a)=-4/5 π/4cos(π/4+b)=12/13 0sin（a+b）=sin（π/4+b-(π/4-a)）=sin(π/4+b)cos(π/4-a)+cos(π/4+b)sin(π/4-a) =5/13*3/5+12/13*(-4/5)= -33/65
