| ( | x | − | 15 | ) | ( | - | 1 2 | x | + | 45 | ) | = | 400 |
| 方程左边 = | x | ( | - | 1 2 | x | + | 45 | ) | − | 15 | ( | - | 1 2 | x | + | 45 | ) |
| = | - | x | × | 1 2 | x | + | x | × | 45 | − | 15 | ( | - | 1 2 | x | + | 45 | ) |
| = | - | x | × | 1 2 | x | + | 45 | x | + | 15 | × | 1 2 | x | − | 15 | × | 45 |
| = | - | x | × | 1 2 | x | + | 45 | x | + | 15 2 | x | − | 675 |
| = | - | x | × | 1 2 | x | + | 105 2 | x | − | 675 |
| - | x | × | 1 2 | x | + | 105 2 | x | − | 675 | = | 400 |