π/4π/2< π/4+a<π => 角π/4+a在第二象限由 cos(π/4+a)= -3/5 => sin(π/4+a)=√[1-(-3/5)^2]=4/50 3π/4由 sin(3π/4+b)=5/13 => cos(3π/4+b)=-√[1-(5/13)^2]=-12/13sin(a+b)=-sin(π+a+b)=-sin[(π/4+a)+(3π/4+b)] =sin(π/4+a)cos(3π/4+b)+cos(π/4+a)sin(3π/4+b) =4/5 * (-12/13) + (-3/5) * (5/13) =-63/65
