| ( | x | − | 10 | ) | ( | 510 | − | 10 | x | + | 200 | ) | = | 7900 |
| 方程左边 = | x | ( | 510 | − | 10 | x | + | 200 | ) | − | 10 | ( | 510 | − | 10 | x | + | 200 | ) |
| = | x | × | 510 | − | x | × | 10 | x | + | x | × | 200 | − | 10 | ( | 510 | − | 10 | x | + | 200 | ) |
| = | 710 | x | − | x | × | 10 | x | − | 10 | ( | 510 | − | 10 | x | + | 200 | ) |
| = | 710 | x | − | x | × | 10 | x | − | 10 | × | 510 | + | 10 | × | 10 | x | − | 10 | × | 200 |
| = | 710 | x | − | x | × | 10 | x | − | 5100 | + | 100 | x | − | 2000 |
| = | 810 | x | − | x | × | 10 | x | − | 7100 |
| 810 | x | − | x | × | 10 | x | − | 7100 | = | 7900 |