| ( | 1 | + | x | ) | ( | 80 | − | 2 | x | ) | − | 1200 | = | 0 |
| 方程左边 = | 1 | ( | 80 | − | 2 | x | ) | + | x | ( | 80 | − | 2 | x | ) | − | 1200 |
| = | 1 | × | 80 | − | 1 | × | 2 | x | + | x | ( | 80 | − | 2 | x | ) | − | 1200 |
| = | 80 | − | 2 | x | + | x | ( | 80 | − | 2 | x | ) | − | 1200 |
| = | - | 1120 | − | 2 | x | + | x | ( | 80 | − | 2 | x | ) |
| = | - | 1120 | − | 2 | x | + | x | × | 80 | − | x | × | 2 | x |
| = | - | 1120 | + | 78 | x | − | x | × | 2 | x |
| - | 1120 | + | 78 | x | − | x | × | 2 | x | = | 0 |