coa（a+B）=3/5sin(a+b)=4/5sin（B-π/4)=5/13cos(B-π/4)=12/13cos（a+π/4）=cos[(a+b)-(b-π/4)]=cos(a+b)cos(b-π/4)+sin(a+b)sin(b-π/4)=3/5(12/13)+(4/5)(5/13)=56/65