( | x | − | 5 | ) | ( | 300 | + | 500 | − | 20 | x | ) | = | 7820 |
方程左边 = | x | ( | 300 | + | 500 | − | 20 | x | ) | − | 5 | ( | 300 | + | 500 | − | 20 | x | ) |
= | x | × | 300 | + | x | × | 500 | − | x | × | 20 | x | − | 5 | ( | 300 | + | 500 | − | 20 | x | ) |
= | 800 | x | − | x | × | 20 | x | − | 5 | ( | 300 | + | 500 | − | 20 | x | ) |
= | 800 | x | − | x | × | 20 | x | − | 5 | × | 300 | − | 5 | × | 500 | + | 5 | × | 20 | x |
= | 800 | x | − | x | × | 20 | x | − | 1500 | − | 2500 | + | 100 | x |
= | 900 | x | − | x | × | 20 | x | − | 4000 |
900 | x | − | x | × | 20 | x | − | 4000 | = | 7820 |