2006DMM-Devil Round-D8

What is the maximum number of pieces that an spherical watermelon can be divided into with four straight planar cuts?

## 题解

interesting problem。

$a_n = a_{n-1} + n$

$a_n = \frac{n * (n-1)}{2}$

$b_n = b_{n-1} + a_{n-1}$