| ( | 12 | + | 3 5 | x | ) | ( | 20 | − | 4 5 | x | ) | = | 144 |
| 方程左边 = | 12 | ( | 20 | − | 4 5 | x | ) | + | 3 5 | x | ( | 20 | − | 4 5 | x | ) |
| = | 12 | × | 20 | − | 12 | × | 4 5 | x | + | 3 5 | x | ( | 20 | − | 4 5 | x | ) |
| = | 240 | − | 48 5 | x | + | 3 5 | x | ( | 20 | − | 4 5 | x | ) |
| = | 240 | − | 48 5 | x | + | 3 5 | x | × | 20 | − | 3 5 | x | × | 4 5 | x |
| = | 240 | − | 48 5 | x | + | 12 | x | − | 12 25 | x | x |
| = | 240 | + | 12 5 | x | − | 12 25 | x | x |
| 240 | + | 12 5 | x | − | 12 25 | x | x | = | 144 |