Questions:
a. The surface Σ is given by z=z(x,y). Derive the area dσ of the lateral surface in terms of dx and dy.
b. Calculate ʃʃ(Σ)(x^2+y^2)dσ, where the surface is given by z=16-x2-y2 lying between x^2+y^2=1 and x^2+y^2=9
圖片參考：https://s.yimg.com/rk/AF02562699/o/966716517.jpg
更新1:
想請問，b小題的第二行變第三行 r^2=x^2+y^2 從x^2+y^2=1積到x^2+y^2=9 dxdy怎麼代換成rdr
