| ( | a | − | 30 | ) | ( | - | 2 | a | + | 100 | ) | = | 200 |
| 方程左边 = | a | ( | - | 2 | a | + | 100 | ) | − | 30 | ( | - | 2 | a | + | 100 | ) |
| = | - | a | × | 2 | a | + | a | × | 100 | − | 30 | ( | - | 2 | a | + | 100 | ) |
| = | - | a | × | 2 | a | + | 100 | a | + | 30 | × | 2 | a | − | 30 | × | 100 |
| = | - | a | × | 2 | a | + | 100 | a | + | 60 | a | − | 3000 |
| = | - | a | × | 2 | a | + | 160 | a | − | 3000 |
| - | a | × | 2 | a | + | 160 | a | − | 3000 | = | 200 |