| ( | 50 | × | 9 10 | − | 40 | ) | ( | 50 | − | a | ) | + | a | ( | 80 | × | 9 10 | − | 65 | ) | = | 0 |
| 方程左边 = | 50 | × | 9 10 | ( | 50 | − | a | ) | − | 40 | ( | 50 | − | a | ) | + | a | ( | 80 | × | 9 10 | − | 65 | ) |
| = | 45 | ( | 50 | − | a | ) | − | 40 | ( | 50 | − | a | ) | + | a | ( | 80 | × | 9 10 | − | 65 | ) |
| = | 45 | × | 50 | − | 45 | a | − | 40 | ( | 50 | − | a | ) | + | a | ( | 80 | × | 9 10 | − | 65 | ) |
| = | 2250 | − | 45 | a | − | 40 | ( | 50 | − | a | ) | + | a | ( | 80 | × | 9 10 | − | 65 | ) |
| = | 2250 | − | 45 | a | − | 40 | × | 50 | + | 40 | a | + | a | ( | 80 | × | 9 10 | − | 65 | ) |
| = | 2250 | − | 45 | a | − | 2000 | + | 40 | a | + | a | ( | 80 | × | 9 10 | − | 65 | ) |
| = | 250 | − | 5 | a | + | a | ( | 80 | × | 9 10 | − | 65 | ) |
| = | 250 | − | 5 | a | + | a | × | 80 | × | 9 10 | − | a | × | 65 |
| = | 250 | − | 5 | a | + | a | × | 72 | − | a | × | 65 |
| = | 250 | + | 2 | a |
| 250 | + | 2 | a | = | 0 |
| 2 | a | = | 0 | − | 250 |
| 2 | a | = | - | 250 |
| a | = | - | 250 | ÷ | 2 |
| = | - | 250 | × | 1 2 |
| = | - | 125 | × | 1 |
| a | = | - | 125 |