Solution
= Solution
$s(t)=\int^t|\dot x(u)|du$, $t=dx/ds$, $\kappa=|dt/ds|$, and $n=\kappa^{-1}dt/ds$. Differentiating $t\cdot t=1$ proves $t\perp n$. Here $|\dot x|=2e^t$ and
$$
t=\tfrac12(\cos t-\sin t,\sin t+\cos t,\sqrt2),\quad n=\frac{(-2\sin t,2\cos t,\sqrt2)}{\sqrt{10}},\quad \kappa=\frac{\sqrt{10}}{4e^t}.
$$