| ( | x | − | 40 | ) | ( | 400 | − | 20 | x | + | 1000 | ) | = | 0 |
| 方程左边 = | x | ( | 400 | − | 20 | x | + | 1000 | ) | − | 40 | ( | 400 | − | 20 | x | + | 1000 | ) |
| = | x | × | 400 | − | x | × | 20 | x | + | x | × | 1000 | − | 40 | ( | 400 | − | 20 | x | + | 1000 | ) |
| = | 1400 | x | − | x | × | 20 | x | − | 40 | ( | 400 | − | 20 | x | + | 1000 | ) |
| = | 1400 | x | − | x | × | 20 | x | − | 40 | × | 400 | + | 40 | × | 20 | x | − | 40 | × | 1000 |
| = | 1400 | x | − | x | × | 20 | x | − | 16000 | + | 800 | x | − | 40000 |
| = | 2200 | x | − | x | × | 20 | x | − | 56000 |
| 2200 | x | − | x | × | 20 | x | − | 56000 | = | 0 |