1)f(x,y)=1/(Pi*R^2)2)f(x)=integal (-sqrt(R^2-x^2),sqrt(R^2-x^2)) (1/(PiR^2))dy=2sqrt(R^2-x^2)/(PiR^2) f(y)同理=2sqrt（R^2-y^2)/(PiR^2)3)f(y|x)=f(x,y)/f(x)=1/(2sqrt(R^2-x^2))
