| ( | 10 | − | x | − | 5 | ) | ( | 500 | + | 200 | x | ) | = | 2800 |
| 方程左边 = | 10 | ( | 500 | + | 200 | x | ) | − | x | ( | 500 | + | 200 | x | ) | − | 5 | ( | 500 | + | 200 | x | ) |
| = | 10 | × | 500 | + | 10 | × | 200 | x | − | x | ( | 500 | + | 200 | x | ) | − | 5 | ( | 500 | + | 200 | x | ) |
| = | 5000 | + | 2000 | x | − | x | ( | 500 | + | 200 | x | ) | − | 5 | ( | 500 | + | 200 | x | ) |
| = | 5000 | + | 2000 | x | − | x | × | 500 | − | x | × | 200 | x | − | 5 | ( | 500 | + | 200 | x | ) |
| = | 5000 | + | 1500 | x | − | x | × | 200 | x | − | 5 | ( | 500 | + | 200 | x | ) |
| = | 5000 | + | 1500 | x | − | x | × | 200 | x | − | 5 | × | 500 | − | 5 | × | 200 | x |
| = | 5000 | + | 1500 | x | − | x | × | 200 | x | − | 2500 | − | 1000 | x |
| = | 2500 | + | 500 | x | − | x | × | 200 | x |
| 2500 | + | 500 | x | − | x | × | 200 | x | = | 2800 |
| x2= | 3 2 |