( | - | 2 | x | + | 800 | ) | ( | x | − | 200 | ) | = | 20000 |
方程左边 = | - | 2 | x | ( | x | − | 200 | ) | + | 800 | ( | x | − | 200 | ) |
= | - | 2 | x | x | + | 2 | x | × | 200 | + | 800 | ( | x | − | 200 | ) |
= | - | 2 | x | x | + | 400 | x | + | 800 | ( | x | − | 200 | ) |
= | - | 2 | x | x | + | 400 | x | + | 800 | x | − | 800 | × | 200 |
= | - | 2 | x | x | + | 400 | x | + | 800 | x | − | 160000 |
= | - | 2 | x | x | + | 1200 | x | − | 160000 |
- | 2 | x | x | + | 1200 | x | − | 160000 | = | 20000 |