| ( | 20 | − | 3 | x | ) | ( | 12 | − | 2 | x | ) | = | 112 |
| 方程左边 = | 20 | ( | 12 | − | 2 | x | ) | − | 3 | x | ( | 12 | − | 2 | x | ) |
| = | 20 | × | 12 | − | 20 | × | 2 | x | − | 3 | x | ( | 12 | − | 2 | x | ) |
| = | 240 | − | 40 | x | − | 3 | x | ( | 12 | − | 2 | x | ) |
| = | 240 | − | 40 | x | − | 3 | x | × | 12 | + | 3 | x | × | 2 | x |
| = | 240 | − | 40 | x | − | 36 | x | + | 6 | x | x |
| = | 240 | − | 76 | x | + | 6 | x | x |
| 240 | − | 76 | x | + | 6 | x | x | = | 112 |
| x2= | 32 3 |