| 3600 | − | 4 | ( | 60 | − | 5 | m | ) | ( | m | + | 15 | ) | = | 0 |
| 方程左边 = | 3600 | − | 4 | × | 60 | ( | m | + | 15 | ) | + | 4 | × | 5 | m | ( | m | + | 15 | ) |
| = | 3600 | − | 240 | ( | m | + | 15 | ) | + | 20 | m | ( | m | + | 15 | ) |
| = | 3600 | − | 240 | m | − | 240 | × | 15 | + | 20 | m | ( | m | + | 15 | ) |
| = | 3600 | − | 240 | m | − | 3600 | + | 20 | m | ( | m | + | 15 | ) |
| = | 0 | − | 240 | m | + | 20 | m | ( | m | + | 15 | ) |
| = | - | 240 | m | + | 20 | m | m | + | 20 | m | × | 15 |
| = | - | 240 | m | + | 20 | m | m | + | 300 | m |
| = | 60 | m | + | 20 | m | m |
| 60 | m | + | 20 | m | m | = | 0 |