| ( | ( | 500 | − | 10 | a | ) | × | 40 | ) | − | ( | ( | 50 | + | a | ) | ( | 500 | − | 10 | a | ) | ) | = | 8000 |
| 方程左边 = | ( | 500 | − | 10 | a | ) | × | 40 | − | ( | ( | 50 | + | a | ) | ( | 500 | − | 10 | a | ) | ) |
| = | 500 | × | 40 | − | 10 | a | × | 40 | − | ( | ( | 50 | + | a | ) | ( | 500 | − | 10 | a | ) | ) |
| = | 20000 | − | 400 | a | − | ( | ( | 50 | + | a | ) | ( | 500 | − | 10 | a | ) | ) |
| = | 20000 | − | 400 | a | − | ( | 50 | + | a | ) | ( | 500 | − | 10 | a | ) |
| = | 20000 | − | 400 | a | − | 50 | ( | 500 | − | 10 | a | ) | − | a | ( | 500 | − | 10 | a | ) |
| = | 20000 | − | 400 | a | − | 50 | × | 500 | + | 50 | × | 10 | a | − | a | ( | 500 | − | 10 | a | ) |
| = | 20000 | − | 400 | a | − | 25000 | + | 500 | a | − | a | ( | 500 | − | 10 | a | ) |
| = | - | 5000 | + | 100 | a | − | a | ( | 500 | − | 10 | a | ) |
| = | - | 5000 | + | 100 | a | − | a | × | 500 | + | a | × | 10 | a |
| = | - | 5000 | − | 400 | a | + | a | × | 10 | a |
| - | 5000 | − | 400 | a | + | a | × | 10 | a | = | 8000 |