| ( | x | − | 30 | ) | ( | 10 | ( | x | − | 400 | ) | ) | = | 10000 |
| 方程左边 = | x | ( | 10 | ( | x | − | 400 | ) | ) | − | 30 | ( | 10 | ( | x | − | 400 | ) | ) |
| = | x | × | 10 | ( | x | − | 400 | ) | − | 30 | ( | 10 | ( | x | − | 400 | ) | ) |
| = | x | × | 10 | x | − | x | × | 10 | × | 400 | − | 30 | ( | 10 | ( | x | − | 400 | ) | ) |
| = | x | × | 10 | x | − | x | × | 4000 | − | 30 | ( | 10 | ( | x | − | 400 | ) | ) |
| = | x | × | 10 | x | − | 4000 | x | − | 30 | × | 10 | ( | x | − | 400 | ) |
| = | x | × | 10 | x | − | 4000 | x | − | 300 | ( | x | − | 400 | ) |
| = | x | × | 10 | x | − | 4000 | x | − | 300 | x | + | 300 | × | 400 |
| = | x | × | 10 | x | − | 4000 | x | − | 300 | x | + | 120000 |
| = | x | × | 10 | x | − | 4300 | x | + | 120000 |
| x | × | 10 | x | − | 4300 | x | + | 120000 | = | 10000 |