( | 6 | − | x | ) | ( | 400 | × | 5 4 | + | 40 | x | ) | = | 1920 |
方程左边 = | 6 | ( | 400 | × | 5 4 | + | 40 | x | ) | − | x | ( | 400 | × | 5 4 | + | 40 | x | ) |
= | 6 | × | 400 | × | 5 4 | + | 6 | × | 40 | x | − | x | ( | 400 | × | 5 4 | + | 40 | x | ) |
= | 3000 | + | 240 | x | − | x | ( | 400 | × | 5 4 | + | 40 | x | ) |
= | 3000 | + | 240 | x | − | x | × | 400 | × | 5 4 | − | x | × | 40 | x |
= | 3000 | + | 240 | x | − | x | × | 500 | − | x | × | 40 | x |
= | 3000 | − | 260 | x | − | x | × | 40 | x |
3000 | − | 260 | x | − | x | × | 40 | x | = | 1920 |