| ( | 8 | + | 1 2 | x | ) | ( | 240 | − | 10 | x | ) | = | 1980 |
| 方程左边 = | 8 | ( | 240 | − | 10 | x | ) | + | 1 2 | x | ( | 240 | − | 10 | x | ) |
| = | 8 | × | 240 | − | 8 | × | 10 | x | + | 1 2 | x | ( | 240 | − | 10 | x | ) |
| = | 1920 | − | 80 | x | + | 1 2 | x | ( | 240 | − | 10 | x | ) |
| = | 1920 | − | 80 | x | + | 1 2 | x | × | 240 | − | 1 2 | x | × | 10 | x |
| = | 1920 | − | 80 | x | + | 120 | x | − | 5 | x | x |
| = | 1920 | + | 40 | x | − | 5 | x | x |
| 1920 | + | 40 | x | − | 5 | x | x | = | 1980 |