| ( | 20 | + | x | ) | ( | 20 | + | x | ) | = | ( | 20 | + | x | ) | x |
| 方程左边 = | 20 | ( | 20 | + | x | ) | + | x | ( | 20 | + | x | ) |
| = | 20 | × | 20 | + | 20 | x | + | x | ( | 20 | + | x | ) |
| = | 400 | + | 20 | x | + | x | ( | 20 | + | x | ) |
| = | 400 | + | 20 | x | + | x | × | 20 | + | x | x |
| = | 400 | + | 40 | x | + | x | x |
| 400 | + | 40 | x | + | x | x | = | ( | 20 | + | x | ) | x |
| 方程右边 = | 20 | x | + | x | x |
| 400 | + | 40 | x | + | x | x | = | 20 | x | + | x | x |