Question 87
The point $K$ on the edge $AB$ of the cube $ABCD A_{1}B_{1}C_{1}D_{1}$ is such that the angle between the line $A_{1}B$ and the plane $(B_{1}CK)$ is equal to $60^{\circ}$. Find $\tan \alpha$, where $\alpha$ is the angle between the planes $(B_{1}CK)$ and $(ABC)$.