| ( | 60 | − | a | − | 40 | ) | ( | 30 | a | + | 300 | ) | = | 6480 |
| 方程左边 = | 60 | ( | 30 | a | + | 300 | ) | − | a | ( | 30 | a | + | 300 | ) | − | 40 | ( | 30 | a | + | 300 | ) |
| = | 60 | × | 30 | a | + | 60 | × | 300 | − | a | ( | 30 | a | + | 300 | ) | − | 40 | ( | 30 | a | + | 300 | ) |
| = | 1800 | a | + | 18000 | − | a | ( | 30 | a | + | 300 | ) | − | 40 | ( | 30 | a | + | 300 | ) |
| = | 1800 | a | + | 18000 | − | a | × | 30 | a | − | a | × | 300 | − | 40 | ( | 30 | a | + | 300 | ) |
| = | 1500 | a | + | 18000 | − | a | × | 30 | a | − | 40 | ( | 30 | a | + | 300 | ) |
| = | 1500 | a | + | 18000 | − | a | × | 30 | a | − | 40 | × | 30 | a | − | 40 | × | 300 |
| = | 1500 | a | + | 18000 | − | a | × | 30 | a | − | 1200 | a | − | 12000 |
| = | 300 | a | + | 6000 | − | a | × | 30 | a |
| 300 | a | + | 6000 | − | a | × | 30 | a | = | 6480 |