| ( | 280 | − | 10 | a | ) | ( | 10 | + | a | ) | = | 0 |
| 方程左边 = | 280 | ( | 10 | + | a | ) | − | 10 | a | ( | 10 | + | a | ) |
| = | 280 | × | 10 | + | 280 | a | − | 10 | a | ( | 10 | + | a | ) |
| = | 2800 | + | 280 | a | − | 10 | a | ( | 10 | + | a | ) |
| = | 2800 | + | 280 | a | − | 10 | a | × | 10 | − | 10 | a | a |
| = | 2800 | + | 280 | a | − | 100 | a | − | 10 | a | a |
| = | 2800 | + | 180 | a | − | 10 | a | a |
| 2800 | + | 180 | a | − | 10 | a | a | = | 0 |