| ( | 12 | − | 2 | x | ) | ( | 20 | − | 2 | x | ) | = | 128 |
| 方程左边 = | 12 | ( | 20 | − | 2 | x | ) | − | 2 | x | ( | 20 | − | 2 | x | ) |
| = | 12 | × | 20 | − | 12 | × | 2 | x | − | 2 | x | ( | 20 | − | 2 | x | ) |
| = | 240 | − | 24 | x | − | 2 | x | ( | 20 | − | 2 | x | ) |
| = | 240 | − | 24 | x | − | 2 | x | × | 20 | + | 2 | x | × | 2 | x |
| = | 240 | − | 24 | x | − | 40 | x | + | 4 | x | x |
| = | 240 | − | 64 | x | + | 4 | x | x |
| 240 | − | 64 | x | + | 4 | x | x | = | 128 |