| 50 | + | 70 | + | ( | 70 | + | x | ) | + | ( | 80 | + | 90 | − | x | ) | = | 360 |
| 方程左边 = | 120 | + | ( | 70 | + | x | ) | + | ( | 80 | + | 90 | − | x | ) |
| 120 | + | ( | 70 | + | x | ) | + | ( | 80 | + | 90 | − | x | ) | = | 360 |
| 方程左边 = | 120 | + | 70 | + | x | + | ( | 80 | + | 90 | − | x | ) |
| = | 190 | + | x | + | ( | 80 | + | 90 | − | x | ) |
| = | 190 | + | x | + | 80 | + | 90 | − | x |
| = | 360 | + | 0 | x |
| 360 | = | 360 |
| = | 360 | − | 360 |