| 8 | ( | 20 | − | x | ) | ( | 4 | + | x | ) | = | 1080 |
| 方程左边 = | 8 | × | 20 | ( | 4 | + | x | ) | − | 8 | x | ( | 4 | + | x | ) |
| = | 160 | ( | 4 | + | x | ) | − | 8 | x | ( | 4 | + | x | ) |
| = | 160 | × | 4 | + | 160 | x | − | 8 | x | ( | 4 | + | x | ) |
| = | 640 | + | 160 | x | − | 8 | x | ( | 4 | + | x | ) |
| = | 640 | + | 160 | x | − | 8 | x | × | 4 | − | 8 | x | x |
| = | 640 | + | 160 | x | − | 32 | x | − | 8 | x | x |
| = | 640 | + | 128 | x | − | 8 | x | x |
| 640 | + | 128 | x | − | 8 | x | x | = | 1080 |