sin（a-b）cos a- cos（b-a）sin a=3/5
即：sin（a-b）cos a- cos（a-b）sin a=3/5
sin(a-b-a)=3/5
得：sinb=-3/5
因为b是第三象限角,所以,cosb=-4/5
所以,sin(b+5π/4)=sinbcos(5π/4)+cosbsin(5π/4)
=(-3/5)(-√2/2)+(-4/5)(-√2/2)
=7√2/10