Let $h: (0,1) \times (0,1)->\mathbb{R}$ be a differentiable function, and let $S=\{(u,v,h(u,v))| u,v \in (0,1)\}$. Determine the differentiable functions $h$ for which the area of $S$ is $1$.
We know that the area of $S$ is $\int_{(0,1) \times (0,1)} ((\dfrac{\partial h}{\partial u})^2+(\dfrac{\partial h}{\partial v})^2+1)^ \frac{1}{2} dudv$ but know how can i resolve the problem?