19. Given $\mathrm{Rt} \triangle A B C$ with the two legs $A C=2$, $C B=3$, and $C P$ as the angle bisector of $\angle A C B$ ($P$ is on the hypotenuse $A B$), fold the triangle along $C P$ to form a dihedral angle $A$ $-C P-B$. When $A U=24 / \sqrt{c}$, the size of the dihedral angle $A-C P$ $-B$ is
19. $\arccos \left(-\frac{1}{6}\right)$