| ( | X | − | 30 | ) | ( | 680 | − | 10 | X | ) | = | 3120 |
| 方程左边 = | X | ( | 680 | − | 10 | X | ) | − | 30 | ( | 680 | − | 10 | X | ) |
| = | X | × | 680 | − | X | × | 10 | X | − | 30 | ( | 680 | − | 10 | X | ) |
| = | 680 | X | − | X | × | 10 | X | − | 30 | × | 680 | + | 30 | × | 10 | X |
| = | 680 | X | − | X | × | 10 | X | − | 20400 | + | 300 | X |
| = | 980 | X | − | X | × | 10 | X | − | 20400 |
| 980 | X | − | X | × | 10 | X | − | 20400 | = | 3120 |