| ( | 200 | − | 20 | x | ) | ( | x | + | 2 | ) | − | 640 | = | 0 |
| 方程左边 = | 200 | ( | x | + | 2 | ) | − | 20 | x | ( | x | + | 2 | ) | − | 640 |
| = | 200 | x | + | 200 | × | 2 | − | 20 | x | ( | x | + | 2 | ) | − | 640 |
| = | 200 | x | + | 400 | − | 20 | x | ( | x | + | 2 | ) | − | 640 |
| = | 200 | x | − | 240 | − | 20 | x | ( | x | + | 2 | ) |
| = | 200 | x | − | 240 | − | 20 | x | x | − | 20 | x | × | 2 |
| = | 200 | x | − | 240 | − | 20 | x | x | − | 40 | x |
| = | 160 | x | − | 240 | − | 20 | x | x |
| 160 | x | − | 240 | − | 20 | x | x | = | 0 |