( | 30 | − | 2 | x | ) | ( | 20 | − | 2 | x | ) | = | 400 |
方程左边 = | 30 | ( | 20 | − | 2 | x | ) | − | 2 | x | ( | 20 | − | 2 | x | ) |
= | 30 | × | 20 | − | 30 | × | 2 | x | − | 2 | x | ( | 20 | − | 2 | x | ) |
= | 600 | − | 60 | x | − | 2 | x | ( | 20 | − | 2 | x | ) |
= | 600 | − | 60 | x | − | 2 | x | × | 20 | + | 2 | x | × | 2 | x |
= | 600 | − | 60 | x | − | 40 | x | + | 4 | x | x |
= | 600 | − | 100 | x | + | 4 | x | x |
600 | − | 100 | x | + | 4 | x | x | = | 400 |