Curves and surfaces

The torus: $\mathbf{r}(\theta, \phi) = ((R + a\cos\theta)\cos\phi, (R + a\cos\theta)\sin\phi, a\sin\theta)$ with $0 < a < R$. (a) Compute $\mathbf{r}_\theta\times\mathbf{r}_\phi$. (b) Find $\mathrm{d}S$. (c) Find the surface area.