| ( | x | − | 30 | ) | ( | 100 | − | 2 | ( | x | − | 30 | ) | ) | = | 3150 |
| 方程左边 = | x | ( | 100 | − | 2 | ( | x | − | 30 | ) | ) | − | 30 | ( | 100 | − | 2 | ( | x | − | 30 | ) | ) |
| = | x | × | 100 | − | x | × | 2 | ( | x | − | 30 | ) | − | 30 | ( | 100 | − | 2 | ( | x | − | 30 | ) | ) |
| = | 100 | x | − | x | × | 2 | x | + | x | × | 2 | × | 30 | − | 30 | ( | 100 | − | 2 | ( | x | − | 30 | ) | ) |
| = | 100 | x | − | x | × | 2 | x | + | x | × | 60 | − | 30 | ( | 100 | − | 2 | ( | x | − | 30 | ) | ) |
| = | 160 | x | − | x | × | 2 | x | − | 30 | ( | 100 | − | 2 | ( | x | − | 30 | ) | ) |
| = | 160 | x | − | x | × | 2 | x | − | 30 | × | 100 | + | 30 | × | 2 | ( | x | − | 30 | ) |
| = | 160 | x | − | x | × | 2 | x | − | 3000 | + | 60 | ( | x | − | 30 | ) |
| = | 160 | x | − | x | × | 2 | x | − | 3000 | + | 60 | x | − | 60 | × | 30 |
| = | 160 | x | − | x | × | 2 | x | − | 3000 | + | 60 | x | − | 1800 |
| = | 220 | x | − | x | × | 2 | x | − | 4800 |
| 220 | x | − | x | × | 2 | x | − | 4800 | = | 3150 |