| ( | 80 | − | 9 | x | ) | ( | 9 | x | + | 8 | ) | = | 0 |
| 方程左边 = | 80 | ( | 9 | x | + | 8 | ) | − | 9 | x | ( | 9 | x | + | 8 | ) |
| = | 80 | × | 9 | x | + | 80 | × | 8 | − | 9 | x | ( | 9 | x | + | 8 | ) |
| = | 720 | x | + | 640 | − | 9 | x | ( | 9 | x | + | 8 | ) |
| = | 720 | x | + | 640 | − | 9 | x | × | 9 | x | − | 9 | x | × | 8 |
| = | 720 | x | + | 640 | − | 81 | x | x | − | 72 | x |
| = | 648 | x | + | 640 | − | 81 | x | x |
| 648 | x | + | 640 | − | 81 | x | x | = | 0 |
| x1= | - | 8 9 |
| x2= | 80 9 |