| 120 | + | 2 | r | + | 120 | + | 1 2 | ( | r | − | 30 | ) | = | 360 |
| 方程左边 = | 240 | + | 2 | r | + | 1 2 | ( | r | − | 30 | ) |
| 240 | + | 2 | r | + | 1 2 | ( | r | − | 30 | ) | = | 360 |
| 方程左边 = | 240 | + | 2 | r | + | 1 2 | r | − | 1 2 | × | 30 |
| = | 240 | + | 2 | r | + | 1 2 | r | − | 15 |
| = | 225 | + | 5 2 | r |
| 225 | + | 5 2 | r | = | 360 |
5 2 | r | = | 360 | − | 225 |
5 2 | r | = | 135 |
| r | = | 135 | ÷ | 5 2 |
| = | 135 | × | 2 5 |
| = | 27 | × | 2 |
| r | = | 54 |