| ( | n | + | 1 | − | 2 | ) | × | 180 | = | ( | n | − | 2 | ) | × | 180 | × | 11 10 |
| Left side of the equation = | n | × | 180 | + | 1 | × | 180 | − | 2 | × | 180 |
| = | n | × | 180 | + | 180 | − | 360 |
| = | 180 | n | − | 180 |
| 180 | n | − | 180 | = | ( | n | − | 2 | ) | × | 180 | × | 11 10 |
| Right side of the equation = | ( | n | − | 2 | ) | × | 198 |
| 180 | n | − | 180 | = | ( | n | − | 2 | ) | × | 198 |
| Right side of the equation = | n | × | 198 | − | 2 | × | 198 |
| = | n | × | 198 | − | 396 |
| 180 | n | − | 180 | = | 198 | n | − | 396 |
| 180 | n | − | 198 | n | = | - | 396 | + | 180 |
| - | 18 | n | = | - | 396 | + | 180 |
| - | 18 | n | = | - | 216 |
| 216 | = | 18 | n |
| 18 | n | = | 216 |
| n | = | 216 | ÷ | 18 |
| = | 216 | × | 1 18 |
| = | 12 | × | 1 |
| n | = | 12 |