| ( | 3 | x | + | 9 | ) | ( | 5 | x | + | 9 | ) | = | 2 | ÷ | 3 |
| 方程左边 = | 3 | x | ( | 5 | x | + | 9 | ) | + | 9 | ( | 5 | x | + | 9 | ) |
| = | 3 | x | × | 5 | x | + | 3 | x | × | 9 | + | 9 | ( | 5 | x | + | 9 | ) |
| = | 15 | x | x | + | 27 | x | + | 9 | ( | 5 | x | + | 9 | ) |
| = | 15 | x | x | + | 27 | x | + | 9 | × | 5 | x | + | 9 | × | 9 |
| = | 15 | x | x | + | 27 | x | + | 45 | x | + | 81 |
| = | 15 | x | x | + | 72 | x | + | 81 |
| 15 | x | x | + | 72 | x | + | 81 | = | 2 | ÷ | 3 |
| 方程右边 = | 2 3 |
| 15 | x | x | + | 72 | x | + | 81 | = | 2 3 |