Forest pieces

You will write functions for the core of a forest cut into pieces, for the forest that has been lost, and for the number of pieces at which the core vanishes.

The idea. A forest of side $10\ \text{km}$ is cut into $k$ by $k$ equal squares. The side of a piece is $10 \div k$. The core share is $\left(\dfrac{s - 2d}{s}\right)^2$ when $s$ is more than $2d$, and zero when it is not.

Your tasks.

1. Finish core_share. It gives the per cent of the forest that is core.
2. Finish forest_lost. It gives the area lost, from the land area and the old and new shares of cover.
3. Finish first_zero. It gives the smallest $k$ at which the core is zero.