| ( | x | + | 3 | ) | ( | 2 | x | + | 5 | ) | − | x | ( | 3 | x | − | 1 | ) | = | 15 |
| 方程左边 = | x | ( | 2 | x | + | 5 | ) | + | 3 | ( | 2 | x | + | 5 | ) | − | x | ( | 3 | x | − | 1 | ) |
| = | x | × | 2 | x | + | x | × | 5 | + | 3 | ( | 2 | x | + | 5 | ) | − | x | ( | 3 | x | − | 1 | ) |
| = | x | × | 2 | x | + | 5 | x | + | 3 | × | 2 | x | + | 3 | × | 5 | − | x | ( | 3 | x | − | 1 | ) |
| = | x | × | 2 | x | + | 5 | x | + | 6 | x | + | 15 | − | x | ( | 3 | x | − | 1 | ) |
| = | x | × | 2 | x | + | 11 | x | + | 15 | − | x | ( | 3 | x | − | 1 | ) |
| = | x | × | 2 | x | + | 11 | x | + | 15 | − | x | × | 3 | x | + | x | × | 1 |
| = | x | × | 2 | x | + | 12 | x | + | 15 | − | x | × | 3 | x |
| x | × | 2 | x | + | 12 | x | + | 15 | − | x | × | 3 | x | = | 15 |