| ( | 30 | − | a | ) | ( | 30 | − | 1 | − | a | ) | = | 812 |
| 方程左边 = | 30 | ( | 30 | − | 1 | − | a | ) | − | a | ( | 30 | − | 1 | − | a | ) |
| = | 30 | × | 30 | − | 30 | × | 1 | − | 30 | a | − | a | ( | 30 | − | 1 | − | a | ) |
| = | 900 | − | 30 | − | 30 | a | − | a | ( | 30 | − | 1 | − | a | ) |
| = | 870 | − | 30 | a | − | a | ( | 30 | − | 1 | − | a | ) |
| = | 870 | − | 30 | a | − | a | × | 30 | + | a | × | 1 | + | a | a |
| = | 870 | − | 59 | a | + | a | a |
| 870 | − | 59 | a | + | a | a | = | 812 |