This is known as the lazy caterer's sequence, the number is $\frac{n(n+1)+2}{2}$. The idea is to use Euler's Formula $E-V+F=2$ and the fact that the number of edges increases $1$ more than the number of vertices in each line.
This is known as the lazy caterer's sequence, the number is $\frac{n(n+1)+2}{2}$. The idea is to use Euler's Formula $E-V+F=2$ and the fact that the number of edges increases $1$ more than the number of vertices in each line.