| ( | 150 | − | x | ) | ( | 500 | − | 2 | x | ) | − | 80 | ( | 500 | − | 2 | x | ) | = | 12000 |
| 方程左边 = | 150 | ( | 500 | − | 2 | x | ) | − | x | ( | 500 | − | 2 | x | ) | − | 80 | ( | 500 | − | 2 | x | ) |
| = | 150 | × | 500 | − | 150 | × | 2 | x | − | x | ( | 500 | − | 2 | x | ) | − | 80 | ( | 500 | − | 2 | x | ) |
| = | 75000 | − | 300 | x | − | x | ( | 500 | − | 2 | x | ) | − | 80 | ( | 500 | − | 2 | x | ) |
| = | 75000 | − | 300 | x | − | x | × | 500 | + | x | × | 2 | x | − | 80 | ( | 500 | − | 2 | x | ) |
| = | 75000 | − | 800 | x | + | x | × | 2 | x | − | 80 | ( | 500 | − | 2 | x | ) |
| = | 75000 | − | 800 | x | + | x | × | 2 | x | − | 80 | × | 500 | + | 80 | × | 2 | x |
| = | 75000 | − | 800 | x | + | x | × | 2 | x | − | 40000 | + | 160 | x |
| = | 35000 | − | 640 | x | + | x | × | 2 | x |
| 35000 | − | 640 | x | + | x | × | 2 | x | = | 12000 |