sin(a-b)cosa-cos(b-a)sina=sin(a-b-a)=sin(-b)=-sin(b)=3/5;sin(b)=-3/5;b是第三象限角,cos(b)=-4/5;sin(b+5pi/4)=-sin(b+pi/4)=-sqrt(2)/2*(sin(b)+cos(b))=7*sqrt(2)/10sqrt表示根号