sin(x-b)cosx-cos(b-x)sinx=sin(x-b)cosx-cos(x-b)sinx=sin[(x-b)-x]
=sin(-b)=-sinb=3/5;
sin(b+5π/4)=sinbcos(5pi/4)+cosbsin5pi/4)=-√2/2*（-3/5-4/5）
=7√2/10