| ( | 8 | + | x | ) | ( | 3 | + | x | ) | − | ( | x | x | ) | = | 90 |
| 方程左边 = | 8 | ( | 3 | + | x | ) | + | x | ( | 3 | + | x | ) | − | ( | x | x | ) |
| = | 8 | × | 3 | + | 8 | x | + | x | ( | 3 | + | x | ) | − | ( | x | x | ) |
| = | 24 | + | 8 | x | + | x | ( | 3 | + | x | ) | − | ( | x | x | ) |
| = | 24 | + | 8 | x | + | x | × | 3 | + | x | x | − | ( | x | x | ) |
| = | 24 | + | 11 | x | + | x | x | − | ( | x | x | ) |
| = | 24 | + | 11 | x | + | x | x | − | x | x |
| 24 | + | 11 | x | + | x | x | − | x | x | = | 90 |