| ( | 2 | x | + | 20 | ) | ( | 100 | − | x | − | 60 | ) | = | 1200 |
| 方程左边 = | 2 | x | ( | 100 | − | x | − | 60 | ) | + | 20 | ( | 100 | − | x | − | 60 | ) |
| = | 2 | x | × | 100 | − | 2 | x | x | − | 2 | x | × | 60 | + | 20 | ( | 100 | − | x | − | 60 | ) |
| = | 200 | x | − | 2 | x | x | − | 120 | x | + | 20 | ( | 100 | − | x | − | 60 | ) |
| = | 80 | x | − | 2 | x | x | + | 20 | ( | 100 | − | x | − | 60 | ) |
| = | 80 | x | − | 2 | x | x | + | 20 | × | 100 | − | 20 | x | − | 20 | × | 60 |
| = | 80 | x | − | 2 | x | x | + | 2000 | − | 20 | x | − | 1200 |
| = | 60 | x | − | 2 | x | x | + | 800 |
| 60 | x | − | 2 | x | x | + | 800 | = | 1200 |