( | x | − | 20 | ) | ( | 1000 | − | 20 | x | ) | = | 4320 |
方程左边 = | x | ( | 1000 | − | 20 | x | ) | − | 20 | ( | 1000 | − | 20 | x | ) |
= | x | × | 1000 | − | x | × | 20 | x | − | 20 | ( | 1000 | − | 20 | x | ) |
= | 1000 | x | − | x | × | 20 | x | − | 20 | × | 1000 | + | 20 | × | 20 | x |
= | 1000 | x | − | x | × | 20 | x | − | 20000 | + | 400 | x |
= | 1400 | x | − | x | × | 20 | x | − | 20000 |
1400 | x | − | x | × | 20 | x | − | 20000 | = | 4320 |