sina=√5/5 sin（a-b）=-√10/10
cosa=2√5/5,cos(a-b)=3√10/10
所以
sinb=sin(a-(a-b))
=sinacos(a-b)-cosasin(a-b)
=√5/5·3√10/10-2√5/5·(-√10/10)
=5√50/50
=√2/2
所以
b=π/4