An equilateral triangle has three equal sides and three equal interior angles. Each angle is 60º or π/3 radians, adding up to 180º or π radians total.

A square has four equal sides and four equal interior angles. Each interior angle is 90º or π/2 radians, adding up to 360º or 2π radians total.

## Interior Angles of Polygons

A short chart of additional polygons with their interior angles provides the following data:**Clearly, an n-sided polygon has a total interior angle measure of**

Pentagon 5 interior angles of 108º (540º or 3π radians total)

Hexagon 6 interior angles of 120º (720º or 4π radians total)

Heptagon 7 interior angles of about 128.57º (900º or 5π radians total)

Octagon 8 interior angles of 135º (1080º or 6π radians total)

**But how do we know, without resorting to a formula, that a triangle possesses an interior angle total of 180º or π radians? And how can we use this example to establish at least one other polygon’s angular total?**

ф = (n-2)π radians or ф = (n-2)180º.

## Determining Interior Angles Total of a Triangle

We begin with drawing a Cartesian Coordinate system with two variables, x and y. We will choose the x-axis (y = 0) as the simplest line for our discussion. The y-axis is perpendicular to it. So the angles between the x-axis and the y-axis for the four quadrants are (by definition) each 90° or π/2 radians. Above y = 0, the angles add up to 180° or π radians. Below that line, the same is true.Now draw a line that cuts across the x-axis at any angle. The figure demonstrates this. The two angles at the intersection above that axis add up to 180° or π radians. So if there are three angles total, say α and β and γ , then α + β + γ = 180°. That being said, note our second image.

## But If All Is Not Clear

But if all of this still leaves you a bit in the dark, see the video below. And even if it doesn’t leave you in the dark, but you wish put an in-depth impress into mind… see the Khan Academy video below.## Consider the Hexagon

Now let us consider a regular hexagon. The hexagon possesses six identical sides, six identical interior angles. According to the formula for the interior angles of a regular polygon, we should arrive at the result of 4π radians. But we don’t have to draw them that way. We can, in fact, take a short cut and arrive at the same answer. Consider our figure featuring the regular hexagon…

**Note:**You might also enjoy Is a Circle a Polygon or Not? Implications for Calculus**← Back to Math-Logic-Design**

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