| ( | 2 | ( | 210 | − | x | ) | ( | x | − | 190 | ) | ) | = | 280 |
| 方程左边 = | 2 | ( | 210 | − | x | ) | ( | x | − | 190 | ) |
| = | 2 | × | 210 | ( | x | − | 190 | ) | − | 2 | x | ( | x | − | 190 | ) |
| = | 420 | ( | x | − | 190 | ) | − | 2 | x | ( | x | − | 190 | ) |
| = | 420 | x | − | 420 | × | 190 | − | 2 | x | ( | x | − | 190 | ) |
| = | 420 | x | − | 79800 | − | 2 | x | ( | x | − | 190 | ) |
| = | 420 | x | − | 79800 | − | 2 | x | x | + | 2 | x | × | 190 |
| = | 420 | x | − | 79800 | − | 2 | x | x | + | 380 | x |
| = | 800 | x | − | 79800 | − | 2 | x | x |
| 800 | x | − | 79800 | − | 2 | x | x | = | 280 |