| ( | 2 | − | 2 | a | ) | ( | 30 | + | 40 | a | ) | = | 70 |
| 方程左边 = | 2 | ( | 30 | + | 40 | a | ) | − | 2 | a | ( | 30 | + | 40 | a | ) |
| = | 2 | × | 30 | + | 2 | × | 40 | a | − | 2 | a | ( | 30 | + | 40 | a | ) |
| = | 60 | + | 80 | a | − | 2 | a | ( | 30 | + | 40 | a | ) |
| = | 60 | + | 80 | a | − | 2 | a | × | 30 | − | 2 | a | × | 40 | a |
| = | 60 | + | 80 | a | − | 60 | a | − | 80 | a | a |
| = | 60 | + | 20 | a | − | 80 | a | a |
| 60 | + | 20 | a | − | 80 | a | a | = | 70 |