| - | 820 | = | - | 3 | n | + | n | ( | n | − | 1 | ) | ÷ | 2 | × | ( | - | 4 | ) |
| 方程右边 = | - | 3 | n | + | n | n | × | 1 2 | ( | - | 4 | ) | − | n | × | 1 | × | 1 2 | ( | - | 4 | ) |
| = | - | 3 | n | + | n | n | × | 1 2 | ( | - | 4 | ) | − | n | × | 1 2 | ( | - | 4 | ) |
| = | - | 3 | n | − | n | n | × | 1 2 | × | 4 | − | n | × | 1 2 | ( | - | 4 | ) |
| = | - | 3 | n | − | n | n | × | 2 | − | n | × | 1 2 | ( | - | 4 | ) |
| = | - | 3 | n | − | n | n | × | 2 | + | n | × | 1 2 | × | 4 |
| = | - | 3 | n | − | n | n | × | 2 | + | n | × | 2 |
| = | - | 1 | n | − | n | n | × | 2 |
| - | 820 | = | - | 1 | n | − | n | n | × | 2 |
| n= | 820 3 |