| ( | 500 | − | 100 | x | ) | ( | 4 | + | x | − | 3 | ) | = | 800 |
| 方程左边 = | 500 | ( | 4 | + | x | − | 3 | ) | − | 100 | x | ( | 4 | + | x | − | 3 | ) |
| = | 500 | × | 4 | + | 500 | x | − | 500 | × | 3 | − | 100 | x | ( | 4 | + | x | − | 3 | ) |
| = | 2000 | + | 500 | x | − | 1500 | − | 100 | x | ( | 4 | + | x | − | 3 | ) |
| = | 500 | + | 500 | x | − | 100 | x | ( | 4 | + | x | − | 3 | ) |
| = | 500 | + | 500 | x | − | 100 | x | × | 4 | − | 100 | x | x | + | 100 | x | × | 3 |
| = | 500 | + | 500 | x | − | 400 | x | − | 100 | x | x | + | 300 | x |
| = | 500 | + | 400 | x | − | 100 | x | x |
| 500 | + | 400 | x | − | 100 | x | x | = | 800 |