cos(π/4一a) = cos(a-π/4) = 3/5 ==> sin(a-π/4) = 4/5 sin(5π/4+b) = -sin(π/4+b) ==> sin(b+π/4) = 12/13；cos(b+π/4) =5/13 sin(a+b)=sin[(a-π/4)+(b+π/4)] = 56/65 /*两角和公式