The answer depends on what information you have about the regular polygon. You need to know, or be able to derive, the number of sides (n) and the length of each side (L). Then the perimeter is n*L units of length.
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The formula used to find the area of any regular polygon is A = 1/2 a P where the lower case a stands for the length of the apothem and the uppercase P stands for the perimeter of the polygon.
The perimeter of a polygon is the sum of the length of each of its sides. If the polygon is a regular polygon the you can calculate the perimeter as [number of sides] *[the length of one side]
A pentagon, which is a 5-sided figure, will have a perimeter equal to the sum of the lengths of all the sides. This will be true for any polygon; the perimeter of any polygon is the sum of the lengths of all the sides. If the pentagon is a regular pentagon, then the perimeter will be 5 times the length of one of the sides. For any regular polygon, the perimeter will be the length of one side times the number of sides in that polygon.
The perimeter is the sum of the sides of a polygon. So, to find the perimeter of a pentagon, you need only to add up the 5 side lengths. In the event the pentagon is regular (all equal sides and angles) then you can multiply the measure of the side by 5 to get the perimeter.
If it is a regular polygon--meaning that all the sides are congruent and all the angles are congruent, then the formula for area of the polygon is A=1/2 ap Here a represents the apothem, which is the distance from the center of the polygon to the midpoint of one side. p represents the perimeter of the polygon found by multiplying one side length by the number of sides. If you only know one variable such as side length, you can find the perimeter and you can find the apothem using trigonomety.