48. In a rectangular parallelepiped $A B C D A_{1} B_{1} C_{1} D_{1}$, it is known that $|A B|=a,|A D|=b,\left|A A_{1}\right|=c$. Find the angle between the planes $A B_{1} D_{1}$ and $A_{1} C_{1} D$.
48. $\arccos \left(\frac{a^{2} b^{2}+b^{2} c^{2}-c^{2} a^{2}}{a^{2} b^{2}+b^{2} c^{2}+c^{2} a^{2}}\right)$.