| ( | 30 | − | 2 | a | + | 10 | ) | ( | a | − | 1 | ) | − | 1 2 | ( | 2 | a | − | 20 | ) | = | 285 |
| 方程左边 = | 30 | ( | a | − | 1 | ) | − | 2 | a | ( | a | − | 1 | ) | + | 10 | ( | a | − | 1 | ) | − | 1 2 | ( | 2 | a | − | 20 | ) |
| = | 30 | a | − | 30 | × | 1 | − | 2 | a | ( | a | − | 1 | ) | + | 10 | ( | a | − | 1 | ) | − | 1 2 | ( | 2 | a | − | 20 | ) |
| = | 30 | a | − | 30 | − | 2 | a | ( | a | − | 1 | ) | + | 10 | ( | a | − | 1 | ) | − | 1 2 | ( | 2 | a | − | 20 | ) |
| = | 30 | a | − | 30 | − | 2 | a | a | + | 2 | a | × | 1 | + | 10 | ( | a | − | 1 | ) | − | 1 2 |
| = | 30 | a | − | 30 | − | 2 | a | a | + | 2 | a | + | 10 | ( | a | − | 1 | ) | − | 1 2 | ( | 2 | a | − | 20 | ) |
| = | 32 | a | − | 30 | − | 2 | a | a | + | 10 | ( | a | − | 1 | ) | − | 1 2 | ( | 2 | a | − | 20 | ) |
| = | 32 | a | − | 30 | − | 2 | a | a | + | 10 | a | − | 10 | × | 1 | − | 1 2 | ( | 2 | a | − | 20 | ) |
| = | 32 | a | − | 30 | − | 2 | a | a | + | 10 | a | − | 10 | − | 1 2 | ( | 2 | a | − | 20 | ) |
| = | 42 | a | − | 40 | − | 2 | a | a | − | 1 2 | ( | 2 | a | − | 20 | ) |
| = | 42 | a | − | 40 | − | 2 | a | a | − | 1 2 | × | 2 | a | + | 1 2 | × | 20 |
| = | 42 | a | − | 40 | − | 2 | a | a | − | 1 | a | + | 10 |
| = | 41 | a | − | 30 | − | 2 | a | a |
| 41 | a | − | 30 | − | 2 | a | a | = | 285 |