| ( | x | − | 8 | ) | ( | 200 | − | 10 | ( | 2 | x | − | 20 | ) | ) | = | 640 |
| 方程左边 = | x | ( | 200 | − | 10 | ( | 2 | x | − | 20 | ) | ) | − | 8 | ( | 200 | − | 10 | ( | 2 | x | − | 20 | ) | ) |
| = | x | × | 200 | − | x | × | 10 | ( | 2 | x | − | 20 | ) | − | 8 | ( | 200 | − | 10 | ( | 2 | x | − | 20 | ) | ) |
| = | 200 | x | − | x | × | 10 | × | 2 | x | + | x | × | 10 | × | 20 | − | 8 | ( | 200 | − | 10 | ( | 2 | x | − | 20 | ) | ) |
| = | 200 | x | − | x | × | 20 | x | + | x | × | 200 | − | 8 | ( | 200 | − | 10 | ( | 2 | x | − | 20 | ) | ) |
| = | 400 | x | − | x | × | 20 | x | − | 8 | ( | 200 | − | 10 | ( | 2 | x | − | 20 | ) | ) |
| = | 400 | x | − | x | × | 20 | x | − | 8 | × | 200 | + | 8 | × | 10 | ( | 2 | x | − | 20 | ) |
| = | 400 | x | − | x | × | 20 | x | − | 1600 | + | 80 | ( | 2 | x | − | 20 | ) |
| = | 400 | x | − | x | × | 20 | x | − | 1600 | + | 80 | × | 2 | x | − | 80 | × | 20 |
| = | 400 | x | − | x | × | 20 | x | − | 1600 | + | 160 | x | − | 1600 |
| = | 560 | x | − | x | × | 20 | x | − | 3200 |
| 560 | x | − | x | × | 20 | x | − | 3200 | = | 640 |