| ( | 30 | − | 2 | a | ) | ( | 12 | − | 2 | a | ) | = | 144 |
| 方程左边 = | 30 | ( | 12 | − | 2 | a | ) | − | 2 | a | ( | 12 | − | 2 | a | ) |
| = | 30 | × | 12 | − | 30 | × | 2 | a | − | 2 | a | ( | 12 | − | 2 | a | ) |
| = | 360 | − | 60 | a | − | 2 | a | ( | 12 | − | 2 | a | ) |
| = | 360 | − | 60 | a | − | 2 | a | × | 12 | + | 2 | a | × | 2 | a |
| = | 360 | − | 60 | a | − | 24 | a | + | 4 | a | a |
| = | 360 | − | 84 | a | + | 4 | a | a |
| 360 | − | 84 | a | + | 4 | a | a | = | 144 |