A frustum and its complete cone

You will write functions for the slant height of a frustum, the height of the complete cone, and the curved surface area. Then you will check the curved surface in a second way, as a big cone minus a small cone.

The idea. The slant height is $l = \sqrt{h^2 + (R - r)^2}$. The complete cone has height $H = \dfrac{hR}{R - r}$ and slant height $L = \dfrac{lR}{R - r}$. The curved surface is $\pi(R + r)l$.

Your tasks.

1. In slant_height, return the slant height of the frustum.
2. In cone_height, return the height of the complete cone.
3. In curved_area, return the curved surface area of the frustum.
4. In curved_by_cones, return the same area as the curved surface of the big cone minus that of the small cone.