| ( | x | − | 40 | ) | ( | 188 | + | 72 | − | x | ) | = | 4000 |
| 方程左边 = | x | ( | 188 | + | 72 | − | x | ) | − | 40 | ( | 188 | + | 72 | − | x | ) |
| = | x | × | 188 | + | x | × | 72 | − | x | x | − | 40 | ( | 188 | + | 72 | − | x | ) |
| = | 260 | x | − | x | x | − | 40 | ( | 188 | + | 72 | − | x | ) |
| = | 260 | x | − | x | x | − | 40 | × | 188 | − | 40 | × | 72 | + | 40 | x |
| = | 260 | x | − | x | x | − | 7520 | − | 2880 | + | 40 | x |
| = | 300 | x | − | x | x | − | 10400 |
| 300 | x | − | x | x | − | 10400 | = | 4000 |