| ( | 8 | + | a | − | 6 | ) | ( | 400 | − | 20 | a | ) | = | 2240 |
| 方程左边 = | 8 | ( | 400 | − | 20 | a | ) | + | a | ( | 400 | − | 20 | a | ) | − | 6 | ( | 400 | − | 20 | a | ) |
| = | 8 | × | 400 | − | 8 | × | 20 | a | + | a | ( | 400 | − | 20 | a | ) | − | 6 | ( | 400 | − | 20 | a | ) |
| = | 3200 | − | 160 | a | + | a | ( | 400 | − | 20 | a | ) | − | 6 | ( | 400 | − | 20 | a | ) |
| = | 3200 | − | 160 | a | + | a | × | 400 | − | a | × | 20 | a | − | 6 | ( | 400 | − | 20 | a | ) |
| = | 3200 | + | 240 | a | − | a | × | 20 | a | − | 6 | ( | 400 | − | 20 | a | ) |
| = | 3200 | + | 240 | a | − | a | × | 20 | a | − | 6 | × | 400 | + | 6 | × | 20 | a |
| = | 3200 | + | 240 | a | − | a | × | 20 | a | − | 2400 | + | 120 | a |
| = | 800 | + | 360 | a | − | a | × | 20 | a |
| 800 | + | 360 | a | − | a | × | 20 | a | = | 2240 |