| ( | 1000 | x | + | 300 | ) | ( | 1 | − | x | ) | = | 420 |
| 方程左边 = | 1000 | x | ( | 1 | − | x | ) | + | 300 | ( | 1 | − | x | ) |
| = | 1000 | x | × | 1 | − | 1000 | x | x | + | 300 | ( | 1 | − | x | ) |
| = | 1000 | x | − | 1000 | x | x | + | 300 | ( | 1 | − | x | ) |
| = | 1000 | x | − | 1000 | x | x | + | 300 | × | 1 | − | 300 | x |
| = | 1000 | x | − | 1000 | x | x | + | 300 | − | 300 | x |
| = | 700 | x | − | 1000 | x | x | + | 300 |
| 700 | x | − | 1000 | x | x | + | 300 | = | 420 |
| x1= | 3 10 |
| x2= | 2 5 |