( | 300 | − | 5 | x | ) | ( | x | − | 30 | ) | = | 1200 |
方程左边 = | 300 | ( | x | − | 30 | ) | − | 5 | x | ( | x | − | 30 | ) |
= | 300 | x | − | 300 | × | 30 | − | 5 | x | ( | x | − | 30 | ) |
= | 300 | x | − | 9000 | − | 5 | x | ( | x | − | 30 | ) |
= | 300 | x | − | 9000 | − | 5 | x | x | + | 5 | x | × | 30 |
= | 300 | x | − | 9000 | − | 5 | x | x | + | 150 | x |
= | 450 | x | − | 9000 | − | 5 | x | x |
450 | x | − | 9000 | − | 5 | x | x | = | 1200 |