sina=√5/5 cosa=2√5/5
sin(a-b)=-√10/10 coss(a-b)=3√10/10
sinb=sin[a-(a-b)]=sinacos(a-b)-cosasin(a-b)=√5/5*3√10/10-2√5/5*(-√10/10)=√2/2
b=45°