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Exploring the Fascinating World of Heptagons

CEO Khai Intela
In the realm of geometry, a heptagon, or septagon, reigns as a captivating seven-sided polygon. The name "heptagon" is a combination of the Latin-derived prefix "septua-" and the Greek suffix "-agon," meaning angle. This unique...

pi /7

In the realm of geometry, a heptagon, or septagon, reigns as a captivating seven-sided polygon. The name "heptagon" is a combination of the Latin-derived prefix "septua-" and the Greek suffix "-agon," meaning angle. This unique polygon possesses a symmetrical charm that is both visually appealing and intellectually stimulating.

Unveiling the Regular Heptagon

Regular heptagon

The regular heptagon is a true marvel. It boasts equal side lengths and angles, with internal angles measuring 5π/7 radians (approximately 128.571 degrees). Represented by the Schläfli symbol {7}, its properties are not only fascinating but also deeply interconnected.

Decoding the Area

The area (A) of a regular heptagon with a side length (a) is given by the formula:

A = (7/4) a^2 cot(π/7) ≈ 3.634 * a^2.

This can be visualized by dividing the unit-sided heptagon into seven triangular "pie slices" and halving each triangle using the apothem as the common side. The apothem, which is half the cotangent of π/7, determines the area of each of the 14 small triangles.

Inscribe, Circumscribe, and Wonder

The area of a regular heptagon inscribed in a circle of radius (R) is given by the formula:

(7R^2/2) * sin(2π/7),

while the area of the circle itself is πR^2. The regular heptagon impressively fills approximately 87.10% of its circumscribed circle.

Craftsmanship in Construction

A neusis construction

Constructing a regular heptagon is a fascinating endeavor. While it cannot be achieved with a compass and straightedge, it can be accomplished using a marked ruler and compass or with the aid of angle trisectors. This unique construction technique is known as a neusis construction and showcases the heptagon's distinct characteristics.

Approximation: A Practical Approach

Approximation example

For practical purposes, we can approximate the side length of a regular heptagon with an error of about 0.2%. In this case, the side length is equivalent to half the side of an equilateral triangle inscribed in the same circle. Although the origins of this approximation are unknown, it has been referenced in several ancient texts and has stood the test of time.

Symmetry: A Thing of Beauty

Symmetries of a regular heptagon

The regular heptagon exhibits exquisite symmetry. It belongs to the D7h point group and possesses a variety of symmetry elements, including rotation axes, mirror planes, and a horizontal mirror plane. The graceful arrangement of these elements adds to the allure of the heptagon.

Diagonals and Heptagonal Triangles

Heptagon with labeled diagonals

The regular heptagon's sides, as well as its shorter and longer diagonals, follow intriguing relationships. These relationships can be expressed through various equations, such as a^2 = c(c - b), b^2 = a(c + a), and c^2 = b(a + b).

Beyond Polygons: Star Heptagons

Star heptagons

Heptagons can also give birth to stunning star heptagons, known as heptagrams. These captivating shapes, represented by Schläfli symbols {7/2} and {7/3}, offer a glimpse into the infinite possibilities within the realm of polygons.

An Enigmatic Presence in Coins and More

The heptagon's allure extends beyond the realm of geometry. It has made its mark in the real world, gracing the designs of numerous coins and objects. From the striking Barbados Dollar and UK 50p and 20p coins to the unique shape of the Brazilian 25-cent coin, heptagons add a touch of elegance to our everyday lives.

The World of Heptagons: A Never-Ending Exploration

As we delve deeper into the enchanting world of heptagons, we realize that their beauty and complexity are boundless. From their unique properties to their presence in various realms of human creativity, heptagons continue to captivate and inspire us. So, let us embrace the allure of the heptagon and embark on an ever-illuminating journey of discovery.


  • Definition and properties of a heptagon with interactive animation
  • Heptagon according to Johnson
  • Another approximate construction method
  • Polygons - Heptagons
  • Recently discovered and highly accurate approximation for the construction of a regular heptagon
  • Heptagon, an approximating construction as an animation
  • A heptagon with a given side, an approximating construction as an animation