As uausl,write the formular of Circumgyrate Surface is not diffcult. But this problem is not easy:

Assume $\displaystyle l_{1}$ and$\displaystyle l_{2}$ are two lines that can not in one plane. and They're not uprightness. Please write the formular of the surface that $\displaystyle l_{2}$ circumgyrated with $\displaystyle l_{1}$.

thanks very much