2. One edge of a tetrahedron is of length $x$, and the other edges are of length 1. If the volume $V$ of the tetrahedron is expressed as a function $f(x)$ of $x$, find the monotonic intervals of the function $f(x)$.
(In the tetrahedron $ABCD$, let the edge $CD=x$, and the other edges are all 1. Let the distance from $D$ to the plane $ABC$ be $H$, then $V=\frac{1}{3} S_{\triangle B C} \cdot H=\frac{\sqrt{3}}{12} H$. Therefore, the monotonic intervals of the functions $V(x)$ and $H(x)$ are the same. Imagine the face $ABD$ rotating around the line $AB$ from the position of the face $ABC$, during this process, $CD$ starts from 0 and gradually increases, examine the change in the distance from point $D$ to the plane $ABC$ to find the answer. Answer: $\left(0, \frac{\sqrt{6}}{2}\right]$ is the increasing interval; $\left[\frac{\sqrt{6}}{2}, \sqrt{3}\right)$ is the decreasing interval. )