| ( | 90 | + | 2 | x | ) | ( | 40 | + | 2 | x | ) | = | 90 | × | 40 | × | 18 25 |
| 方程左边 = | 90 | ( | 40 | + | 2 | x | ) | + | 2 | x | ( | 40 | + | 2 | x | ) |
| = | 90 | × | 40 | + | 90 | × | 2 | x | + | 2 | x | ( | 40 | + | 2 | x | ) |
| = | 3600 | + | 180 | x | + | 2 | x | ( | 40 | + | 2 | x | ) |
| = | 3600 | + | 180 | x | + | 2 | x | × | 40 | + | 2 | x | × | 2 | x |
| = | 3600 | + | 180 | x | + | 80 | x | + | 4 | x | x |
| = | 3600 | + | 260 | x | + | 4 | x | x |
| 3600 | + | 260 | x | + | 4 | x | x | = | 90 | × | 40 | × | 18 25 |
| 方程右边 = | 2592 |
| 3600 | + | 260 | x | + | 4 | x | x | = | 2592 |