Function polygonA

polygonA(n: number, s: number, a: number): number
Calculates the area of a regular polygon.
Parameters
- n: number
  The number of sides
- s: number
  The side length
- a: number
  The apothem length
Returns number
Example
```
// Returns 100
const area = Chalkboard.geom.polygonA(4, 10, 5);
```
- Defined in Chalkboard.geom.ts:288