( | 100 | − | 2 | x | ) | ( | 40 | − | 2 | x | ) | = | 1600 |
方程左边 = | 100 | ( | 40 | − | 2 | x | ) | − | 2 | x | ( | 40 | − | 2 | x | ) |
= | 100 | × | 40 | − | 100 | × | 2 | x | − | 2 | x | ( | 40 | − | 2 | x | ) |
= | 4000 | − | 200 | x | − | 2 | x | ( | 40 | − | 2 | x | ) |
= | 4000 | − | 200 | x | − | 2 | x | × | 40 | + | 2 | x | × | 2 | x |
= | 4000 | − | 200 | x | − | 80 | x | + | 4 | x | x |
= | 4000 | − | 280 | x | + | 4 | x | x |
4000 | − | 280 | x | + | 4 | x | x | = | 1600 |