9. As shown in the figure, points $P_{1}, P_{2}, \cdots P_{10}$ are the midpoints of the vertices or edges of a tetrahedron. The number of quadruplets of points $\left(P_{1}, P_{i}, P_{j}, P_{k}\right)(1<i<j<k \leqslant 10)$ that lie on the same plane is. $\qquad$
33
9. 【Analysis and Solution】First, on each side except point $P_{1}$, there are five points. Any three of these points, when combined with point $P_{1}$, form a group of four points that lie on the same plane. The number of such groups of three points is $\mathrm{C}_{5}^{3}$, and for the three sides, there are $3 \mathrm{C}_{5}^{3}$ such groups.

Second, for each edge containing $P_{1}$, the group of three points on this edge, when combined with the midpoint of the edge on the opposite face, also form a group of four points that lie on the same plane. There are 3 such groups of four points.
In total, there are $\mathrm{C}_{5}^{3}+3=33$ groups.