I will not try to guess what that diameter represents, so I will give you all mensuration formulas for the area of a regular polygon.
s is the measure of the side,
r the radius of the inscribed circle,
R the radius of the circumscribed circle, and
n the number of sides.
If the angle unit in your calculator is the degree, use 180 instead of Pi. Use the formulas with PI if angle unit is set to radians.
In the formulas on the last line of the display screen (the formulas with sine functions) the radius r should be R (circumscribed circle radius). cot is the reciprocal of the tangent function
(cot(x)=1/tan(x) )
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