| ( | 4 | + | 1 2 | x | ) | ( | 20000 | − | 1000 | x | ) | = | 0 |
| 方程左边 = | 4 | ( | 20000 | − | 1000 | x | ) | + | 1 2 | x | ( | 20000 | − | 1000 | x | ) |
| = | 4 | × | 20000 | − | 4 | × | 1000 | x | + | 1 2 | x | ( | 20000 | − | 1000 | x | ) |
| = | 80000 | − | 4000 | x | + | 1 2 | x | ( | 20000 | − | 1000 | x | ) |
| = | 80000 | − | 4000 | x | + | 1 2 | x | × | 20000 | − | 1 2 | x | × | 1000 | x |
| = | 80000 | − | 4000 | x | + | 10000 | x | − | 500 | x | x |
| = | 80000 | + | 6000 | x | − | 500 | x | x |
| 80000 | + | 6000 | x | − | 500 | x | x | = | 0 |