Solution
= Solution
The <proper time> elapsed along the smooth <timelike curve> is
$$
\boxed{T[\gamma]=\int_0^1
\sqrt{-g_{\mu\nu}(x(s))\frac{dx^\mu}{ds}\frac{dx^\nu}{ds}}\,ds}.
$$
The expression is invariant under every orientation-preserving <reparametrization> of the curve.