Formula

Using length of a side :
Area of Polygon = ((side)² * N) / (4Tan(π / N))
Perimeter of Polygon = N * (side)

Using radius (circumradius) :
Area of Polygon = ½ * R² * Sin(2π / N)

Using apothem (inradius) :
Area of Polygon = A² * N * Tan(π / N)
where A = R * Cos(π / N)

Using apothem and length of a side :
Area of Polygon = (A * P) / 2
where A = side / (2 * Tan (π / N))

where,
N = Number of sides, A = Apothem, R = Radius, P = Perimeter