| ( | 30 | + | m | ) | ( | 600 | + | 200 | m | ) | = | 0 |
| 方程左边 = | 30 | ( | 600 | + | 200 | m | ) | + | m | ( | 600 | + | 200 | m | ) |
| = | 30 | × | 600 | + | 30 | × | 200 | m | + | m | ( | 600 | + | 200 | m | ) |
| = | 18000 | + | 6000 | m | + | m | ( | 600 | + | 200 | m | ) |
| = | 18000 | + | 6000 | m | + | m | × | 600 | + | m | × | 200 | m |
| = | 18000 | + | 6600 | m | + | m | × | 200 | m |
| 18000 | + | 6600 | m | + | m | × | 200 | m | = | 0 |