( | 20 | + | 2 | x | ) | ( | 120 | − | x | − | 80 | ) | = | 1200 |
方程左边 = | 20 | ( | 120 | − | x | − | 80 | ) | + | 2 | x | ( | 120 | − | x | − | 80 | ) |
= | 20 | × | 120 | − | 20 | x | − | 20 | × | 80 | + | 2 | x | ( | 120 | − | x | − | 80 | ) |
= | 2400 | − | 20 | x | − | 1600 | + | 2 | x | ( | 120 | − | x | − | 80 | ) |
= | 800 | − | 20 | x | + | 2 | x | ( | 120 | − | x | − | 80 | ) |
= | 800 | − | 20 | x | + | 2 | x | × | 120 | − | 2 | x | x | − | 2 | x | × | 80 |
= | 800 | − | 20 | x | + | 240 | x | − | 2 | x | x | − | 160 | x |
= | 800 | + | 60 | x | − | 2 | x | x |
800 | + | 60 | x | − | 2 | x | x | = | 1200 |