设x=arc tan 1/3
tgx=1/3
[万能公式]
sin2x=2tgx/(1+(tgx)^2)=2/3/(1+1/9)=3/5
sin(arc tan 1/3)*cos(arc tan 1/3)
=sinx*cosx
=sin2x/2
=3/5/2
=3/10