| ( | 60 | − | 2 | a | ) | ( | 40 | − | 2 | a | ) | = | 0 |
| 方程左边 = | 60 | ( | 40 | − | 2 | a | ) | − | 2 | a | ( | 40 | − | 2 | a | ) |
| = | 60 | × | 40 | − | 60 | × | 2 | a | − | 2 | a | ( | 40 | − | 2 | a | ) |
| = | 2400 | − | 120 | a | − | 2 | a | ( | 40 | − | 2 | a | ) |
| = | 2400 | − | 120 | a | − | 2 | a | × | 40 | + | 2 | a | × | 2 | a |
| = | 2400 | − | 120 | a | − | 80 | a | + | 4 | a | a |
| = | 2400 | − | 200 | a | + | 4 | a | a |
| 2400 | − | 200 | a | + | 4 | a | a | = | 0 |