| ( | x | + | 1 | ) | ( | 500 | − | 100 | x | ) | = | 800 |
| 方程左边 = | x | ( | 500 | − | 100 | x | ) | + | 1 | ( | 500 | − | 100 | x | ) |
| = | x | × | 500 | − | x | × | 100 | x | + | 1 | ( | 500 | − | 100 | x | ) |
| = | 500 | x | − | x | × | 100 | x | + | 1 | × | 500 | − | 1 | × | 100 | x |
| = | 500 | x | − | x | × | 100 | x | + | 500 | − | 100 | x |
| = | 400 | x | − | x | × | 100 | x | + | 500 |
| 400 | x | − | x | × | 100 | x | + | 500 | = | 800 |