( | 20 | + | a | ) | ( | 500 | − | 20 | a | ) | − | 16000 | = | ( | 20 | + | a | ) | × | 400 |
方程左边 = | 20 | ( | 500 | − | 20 | a | ) | + | a | ( | 500 | − | 20 | a | ) | − | 16000 |
= | 20 | × | 500 | − | 20 | × | 20 | a | + | a | ( | 500 | − | 20 | a | ) | − | 16000 |
= | 10000 | − | 400 | a | + | a | ( | 500 | − | 20 | a | ) | − | 16000 |
= | - | 6000 | − | 400 | a | + | a | ( | 500 | − | 20 | a | ) |
= | - | 6000 | − | 400 | a | + | a | × | 500 | − | a | × | 20 | a |
= | - | 6000 | + | 100 | a | − | a | × | 20 | a |
- | 6000 | + | 100 | a | − | a | × | 20 | a | = | ( | 20 | + | a | ) | × | 400 |
方程右边 = | 20 | × | 400 | + | a | × | 400 |
= | 8000 | + | a | × | 400 |
- | 6000 | + | 100 | a | − | a | × | 20 | a | = | 8000 | + | 400 | a |