( | 680 | − | 10 | x | ) | ( | x | − | 30 | ) | = | 3700 |
方程左边 = | 680 | ( | x | − | 30 | ) | − | 10 | x | ( | x | − | 30 | ) |
= | 680 | x | − | 680 | × | 30 | − | 10 | x | ( | x | − | 30 | ) |
= | 680 | x | − | 20400 | − | 10 | x | ( | x | − | 30 | ) |
= | 680 | x | − | 20400 | − | 10 | x | x | + | 10 | x | × | 30 |
= | 680 | x | − | 20400 | − | 10 | x | x | + | 300 | x |
= | 980 | x | − | 20400 | − | 10 | x | x |
980 | x | − | 20400 | − | 10 | x | x | = | 3700 |