| ( | y | − | 30 | ) | ( | - | 10 | y | + | 1000 | ) | = | 10000 |
| 方程左边 = | y | ( | - | 10 | y | + | 1000 | ) | − | 30 | ( | - | 10 | y | + | 1000 | ) |
| = | - | y | × | 10 | y | + | y | × | 1000 | − | 30 | ( | - | 10 | y | + | 1000 | ) |
| = | - | y | × | 10 | y | + | 1000 | y | + | 30 | × | 10 | y | − | 30 | × | 1000 |
| = | - | y | × | 10 | y | + | 1000 | y | + | 300 | y | − | 30000 |
| = | - | y | × | 10 | y | + | 1300 | y | − | 30000 |
| - | y | × | 10 | y | + | 1300 | y | − | 30000 | = | 10000 |