| ( | 4 | − | 2 | − | x | ) | ( | 50 | + | 200 | x | ) | = | 300 |
| 方程左边 = | 4 | ( | 50 | + | 200 | x | ) | − | 2 | ( | 50 | + | 200 | x | ) | − | x | ( | 50 | + | 200 | x | ) |
| = | 4 | × | 50 | + | 4 | × | 200 | x | − | 2 | ( | 50 | + | 200 | x | ) | − | x | ( | 50 | + | 200 | x | ) |
| = | 200 | + | 800 | x | − | 2 | ( | 50 | + | 200 | x | ) | − | x | ( | 50 | + | 200 | x | ) |
| = | 200 | + | 800 | x | − | 2 | × | 50 | − | 2 | × | 200 | x | − | x | ( | 50 | + | 200 | x | ) |
| = | 200 | + | 800 | x | − | 100 | − | 400 | x | − | x | ( | 50 | + | 200 | x | ) |
| = | 100 | + | 400 | x | − | x | ( | 50 | + | 200 | x | ) |
| = | 100 | + | 400 | x | − | x | × | 50 | − | x | × | 200 | x |
| = | 100 | + | 350 | x | − | x | × | 200 | x |
| 100 | + | 350 | x | − | x | × | 200 | x | = | 300 |