由z=√(x^2+y^2),设x=zcosu,y=zsinu,z>=0,代入x^2+y^2=2x,得z=2cosu,-π/2<=u<=π/2，∴x=2(cosu)^2=1+cos2u,y=2cosusinu=sin2u,∴两曲面交线在xoy平面的投影是圆
