You calculate the area of the base of a pyramid by using the formula that applies to the given shape of the base.
It’s always possible to divide the base into triangle, and then calculate the area of each triangle.
If the base is a n-sided polygon with equal sides and angles, you can also use the formula for area of regular polygons.
Where A is the area, n is the number of sides, and b is the side length.
Use the following formula instead, if your calculator hasn't got the cotangent feature.