A regular pentagon is simply called a regular pentagon.
Here’s the distinction:
- Pentagon: Any 5-sided polygon.
- Regular pentagon: A pentagon with all five sides equal and all five interior angles equal (each interior angle is 108°).
Unlike some other shapes (for example, a regular 4-sided polygon is commonly called a square), there isn’t a special, widely used name for a regular pentagon beyond regular pentagon.
Why term regular used?
The word “regular” is used in geometry to mean uniform or all the same.
A regular polygon has two properties:
- All sides are equal in length.
- All interior angles are equal.
The term comes from the Latin word regularis, meaning “according to a rule” or “uniform.”
For example:
- A pentagon is any 5-sided polygon. Its sides and angles can be different.
- A regular pentagon has 5 equal sides and 5 equal angles (each angle is 108°).
Similarly:
- A hexagon can have unequal sides and angles.
- A regular hexagon has 6 equal sides and 6 equal angles (each angle is 120°).
So, “regular” distinguishes the special, perfectly symmetrical version of a polygon from the general case. Not every pentagon, hexagon, or octagon is regular, but every regular polygon is both equilateral (equal sides) and equiangular (equal angles).
Conclusion:
A regular polygon is called “regular” because it follows a uniform rule: all its sides are equal and all its angles are equal. Therefore, a regular pentagon is a pentagon that is perfectly symmetrical, with five equal sides and five equal angles. The word “regular” distinguishes this special, uniform shape from an ordinary pentagon, whose sides and angles may differ.
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