| ( | x | − | 4 | ) | ( | 900 | − | 100 | x | ) | = | 800 |
| 方程左边 = | x | ( | 900 | − | 100 | x | ) | − | 4 | ( | 900 | − | 100 | x | ) |
| = | x | × | 900 | − | x | × | 100 | x | − | 4 | ( | 900 | − | 100 | x | ) |
| = | 900 | x | − | x | × | 100 | x | − | 4 | × | 900 | + | 4 | × | 100 | x |
| = | 900 | x | − | x | × | 100 | x | − | 3600 | + | 400 | x |
| = | 1300 | x | − | x | × | 100 | x | − | 3600 |
| 1300 | x | − | x | × | 100 | x | − | 3600 | = | 800 |