设sina=1/3=x/y,即y=3x,
so tana=x/{根号下（y方-x方）},将y代入前式即tana=x方/根号下【（3x）方-x方】}=x方/（根号下9x方-x方）=x方/根号下（8x方）=1/根号8=1/(2倍根号2)=(根号2)/4