I need to show that for $F(x_1, .., x_n)$, the Legendre transformation is,
$$G(s_1, ..., s_n) = \sum_{i}^{N} x_i s_i - F$$
where $$s_i = \frac{\partial F}{\partial x_i}$$ and has the property that $$x_i = \frac{\partial G}{\partial s_i}$$
However, doesn't this come from the definition itself? I mean for one variable, the transformation is, $$G(s) = s x(s) - F(x(s))$$ such that $x(s) = dG/ds$. So how do I prove the above statement?