| ( | x | − | 30 | ) | ( | ( | 40 | − | x | ) | x | × | 200 | + | 600 | ) | = | 8400 |
| 方程左边 = | x | ( | ( | 40 | − | x | ) | x | × | 200 | + | 600 | ) | − | 30 | ( | ( | 40 | − | x | ) | x | × | 200 | + | 600 | ) |
| = | x | ( | 40 | − | x | ) | x | × | 200 | + | x | × | 600 | − | 30 | ( | ( | 40 | − | x | ) | x | × | 200 | + | 600 | ) |
| = | x | × | 40 | x | × | 200 | − | x | x | x | × | 200 | + | 600 | x | − | 30 | ( | ( | 40 | − | x | ) | x | × | 200 | + | 600 | ) |
| = | x | × | 8000 | x | − | x | x | x | × | 200 | + | 600 | x | − | 30 | ( | ( | 40 | − | x | ) | x | × | 200 | + | 600 | ) |
| = | x | × | 8000 | x | − | x | x | x | × | 200 | + | 600 | x | − | 30 | ( | 40 | − | x | ) | x |
| = | x | × | 8000 | x | − | x | x | x | × | 200 | + | 600 | x | − | 6000 | ( | 40 | − | x | ) | x |
| = | x | × | 8000 | x | − | x | x | x | × | 200 | + | 600 | x | − | 6000 | × | 40 | x |
| = | x | × | 8000 | x | − | x | x | x | × | 200 | + | 600 | x | − | 240000 | x | + | 6000 |
| = | x | × | 8000 | x | − | x | x | x | × | 200 | − | 239400 | x | + | 6000 | x | x |
| x | × | 8000 | x | − | x | x | x | × | 200 | − | 239400 | x | + | 6000 | x | x | = | 8400 |