| ( | 50 | − | x | ) | ( | 0 | + | 40 | x | + | 400 | ) | = | 8840 |
| 方程左边 = | 50 | ( | 0 | + | 40 | x | + | 400 | ) | − | x | ( | 0 | + | 40 | x | + | 400 | ) |
| = | 50 | × | 0 | + | 50 | × | 40 | x | + | 50 | × | 400 | − | x | ( | 0 | + | 40 | x | + | 400 | ) |
| = | 0 | + | 2000 | x | + | 20000 | − | x | ( | 0 | + | 40 | x | + | 400 | ) |
| = | 20000 | + | 2000 | x | − | x | ( | 0 | + | 40 | x | + | 400 | ) |
| = | 20000 | + | 2000 | x | − | x | × | 0 | − | x | × | 40 | x | − | x | × | 400 |
| = | 20000 | + | 1600 | x | − | x | × | 40 | x |
| 20000 | + | 1600 | x | − | x | × | 40 | x | = | 8840 |