( | m | − | 30 | ) | ( | 600 | − | 10 | m | ) | = | 10000 |
方程左边 = | m | ( | 600 | − | 10 | m | ) | − | 30 | ( | 600 | − | 10 | m | ) |
= | m | × | 600 | − | m | × | 10 | m | − | 30 | ( | 600 | − | 10 | m | ) |
= | 600 | m | − | m | × | 10 | m | − | 30 | × | 600 | + | 30 | × | 10 | m |
= | 600 | m | − | m | × | 10 | m | − | 18000 | + | 300 | m |
= | 900 | m | − | m | × | 10 | m | − | 18000 |
900 | m | − | m | × | 10 | m | − | 18000 | = | 10000 |