| ( | 3 | Q | + | 20 | ) | ( | Q | − | 20 | ) | = | 0 |
| 方程左边 = | 3 | Q | ( | Q | − | 20 | ) | + | 20 | ( | Q | − | 20 | ) |
| = | 3 | Q | Q | − | 3 | Q | × | 20 | + | 20 | ( | Q | − | 20 | ) |
| = | 3 | Q | Q | − | 60 | Q | + | 20 | ( | Q | − | 20 | ) |
| = | 3 | Q | Q | − | 60 | Q | + | 20 | Q | − | 20 | × | 20 |
| = | 3 | Q | Q | − | 60 | Q | + | 20 | Q | − | 400 |
| = | 3 | Q | Q | − | 40 | Q | − | 400 |
| 3 | Q | Q | − | 40 | Q | − | 400 | = | 0 |
| Q1= | - | 20 3 |
| Q2=20 |