| ( | 5 | − | a | ) | ( | 80 | − | 5 | a | ) | + | ( | 5 2 | − | 1 2 | a | ) | × | 80 | = | 0 |
| 方程左边 = | 5 | ( | 80 | − | 5 | a | ) | − | a | ( | 80 | − | 5 | a | ) | + | ( | 5 2 | − | 1 2 | a | ) | × | 80 |
| = | 5 | × | 80 | − | 5 | × | 5 | a | − | a | ( | 80 | − | 5 | a | ) | + | ( | 5 2 | − | 1 2 | a | ) | × | 80 |
| = | 400 | − | 25 | a | − | a | ( | 80 | − | 5 | a | ) | + | ( | 5 2 | − | 1 2 | a | ) | × | 80 |
| = | 400 | − | 25 | a | − | a | × | 80 | + | a | × | 5 | a | + | ( | 5 2 | − | 1 2 | a | ) | × | 80 |
| = | 400 | − | 105 | a | + | a | × | 5 | a | + | ( | 5 2 | − | 1 2 | a | ) | × | 80 |
| = | 400 | − | 105 | a | + | a | × | 5 | a | + | 5 2 | × | 80 | − | 1 2 | a | × | 80 |
| = | 400 | − | 105 | a | + | a | × | 5 | a | + | 200 | − | 40 | a |
| = | 600 | − | 145 | a | + | a | × | 5 | a |
| 600 | − | 145 | a | + | a | × | 5 | a | = | 0 |