1800 | = | ( | x | − | 4 | ) | ( | - | 50 | x | + | 1200 | ) |
方程右边 = | x | ( | - | 50 | x | + | 1200 | ) | − | 4 | ( | - | 50 | x | + | 1200 | ) |
= | - | x | × | 50 | x | + | x | × | 1200 | − | 4 | ( | - | 50 | x | + | 1200 | ) |
= | - | x | × | 50 | x | + | 1200 | x | + | 4 | × | 50 | x | − | 4 | × | 1200 |
= | - | x | × | 50 | x | + | 1200 | x | + | 200 | x | − | 4800 |
= | - | x | × | 50 | x | + | 1400 | x | − | 4800 |
1800 | = | - | x | × | 50 | x | + | 1400 | x | − | 4800 |