| ( | 9 | x | + | 8 | ) | ( | 80 | − | 9 | x | ) | = | 1855 |
| 方程左边 = | 9 | x | ( | 80 | − | 9 | x | ) | + | 8 | ( | 80 | − | 9 | x | ) |
| = | 9 | x | × | 80 | − | 9 | x | × | 9 | x | + | 8 | ( | 80 | − | 9 | x | ) |
| = | 720 | x | − | 81 | x | x | + | 8 | ( | 80 | − | 9 | x | ) |
| = | 720 | x | − | 81 | x | x | + | 8 | × | 80 | − | 8 | × | 9 | x |
| = | 720 | x | − | 81 | x | x | + | 640 | − | 72 | x |
| = | 648 | x | − | 81 | x | x | + | 640 |
| 648 | x | − | 81 | x | x | + | 640 | = | 1855 |