| ( | 15 | + | a | ) | ( | 20 | − | 15 | − | a | ) | = | 150 |
| 方程左边 = | 15 | ( | 20 | − | 15 | − | a | ) | + | a | ( | 20 | − | 15 | − | a | ) |
| = | 15 | × | 20 | − | 15 | × | 15 | − | 15 | a | + | a | ( | 20 | − | 15 | − | a | ) |
| = | 300 | − | 225 | − | 15 | a | + | a | ( | 20 | − | 15 | − | a | ) |
| = | 75 | − | 15 | a | + | a | ( | 20 | − | 15 | − | a | ) |
| = | 75 | − | 15 | a | + | a | × | 20 | − | a | × | 15 | − | a | a |
| = | 75 | − | 10 | a | − | a | a |
| 75 | − | 10 | a | − | a | a | = | 150 |