| ( | x | − | 30 | ) | ( | 600 | − | 10 | x | + | 400 | ) | = | 10000 |
| 方程左边 = | x | ( | 600 | − | 10 | x | + | 400 | ) | − | 30 | ( | 600 | − | 10 | x | + | 400 | ) |
| = | x | × | 600 | − | x | × | 10 | x | + | x | × | 400 | − | 30 | ( | 600 | − | 10 | x | + | 400 | ) |
| = | 1000 | x | − | x | × | 10 | x | − | 30 | ( | 600 | − | 10 | x | + | 400 | ) |
| = | 1000 | x | − | x | × | 10 | x | − | 30 | × | 600 | + | 30 | × | 10 | x | − | 30 | × | 400 |
| = | 1000 | x | − | x | × | 10 | x | − | 18000 | + | 300 | x | − | 12000 |
| = | 1300 | x | − | x | × | 10 | x | − | 30000 |
| 1300 | x | − | x | × | 10 | x | − | 30000 | = | 10000 |