( | x | − | 30 | ) | ( | - | 2 | x | + | 120 | ) | = | 800 |
方程左边 = | x | ( | - | 2 | x | + | 120 | ) | − | 30 | ( | - | 2 | x | + | 120 | ) |
= | - | x | × | 2 | x | + | x | × | 120 | − | 30 | ( | - | 2 | x | + | 120 | ) |
= | - | x | × | 2 | x | + | 120 | x | + | 30 | × | 2 | x | − | 30 | × | 120 |
= | - | x | × | 2 | x | + | 120 | x | + | 60 | x | − | 3600 |
= | - | x | × | 2 | x | + | 180 | x | − | 3600 |
- | x | × | 2 | x | + | 180 | x | − | 3600 | = | 800 |