\( \int\int_{S}f(x,y,z)dS=\int\int_{D}f(x,y,z)\sqrt{1+g_x^2+g_y^2}dA\)
(upward orientation or normal changes sign)
\( \int\int_{S}F(x,y,z)\cdot dS=\int\int_{D}F(x,y,z) \cdot \langle
-g_x, -g_y, 1\rangle dA\)
(simple surface)
\( \int\int_{S}F(x,y,z)\cdot dS=\int\int_{D}F(x,y,z) \cdot ndA\)