| ( | - | 2 | a | + | 100 | ) | ( | a | − | 30 | ) | = | 150 |
| 方程左边 = | - | 2 | a | ( | a | − | 30 | ) | + | 100 | ( | a | − | 30 | ) |
| = | - | 2 | a | a | + | 2 | a | × | 30 | + | 100 | ( | a | − | 30 | ) |
| = | - | 2 | a | a | + | 60 | a | + | 100 | ( | a | − | 30 | ) |
| = | - | 2 | a | a | + | 60 | a | + | 100 | a | − | 100 | × | 30 |
| = | - | 2 | a | a | + | 60 | a | + | 100 | a | − | 3000 |
| = | - | 2 | a | a | + | 160 | a | − | 3000 |
| - | 2 | a | a | + | 160 | a | − | 3000 | = | 150 |