by Dr. Carol JVF Burns (website creator)
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We'll start by talking about triangles, and then extend the ideas to any polygon.

Recall that lowercase Greek letters are commonly used to denote angles:
this lesson uses $\,\alpha\,$ (alpha), $\,\beta\,$ (beta), and $\,\gamma\,$ (gamma).

It's easy to see that the sum of the angles in any triangle is $\,180^{\circ}\,$, as follows:

First, start with a right triangle (i.e., a triangle that has a $\,90^{\circ}\,$ angle).
Make it into a rectangle, as shown below, to see that $\,\alpha +\beta = 90^{\circ}\,$.
Thus, in any right triangle, the remaining two angles sum to $\,90^{\circ}\,$.

a right triangle a right triangle made into a rectangle

If you have trouble seeing the rectangle that is formed above, then study the following, paying attention to angle $\,\alpha \,$:

  • Start with triangle (1).
  • Slide it to the right, to get (2).
  • Reflect about the horizontal line $\,h\,$, to get (3).
  • Reflect about the vertical line $\,v\,$, to get (4).
  • Slide it down, to get (5).
making a rectangle from a right triangle

Now, consider any triangle.
Drop a perpendicular, as shown at right, bringing two right triangles into the picture.

(Choose a vertex so that the perpendicular is inside the triangle;
such a vertex always exists.)

Thus, we have $\,\alpha + \gamma_1 =90^{\circ}\,$ and $\,\beta + \gamma_2 = 90^{\circ}\,$.

Together, $\,\alpha + \gamma_1 + \gamma_2 + \beta = 180^{\circ}\,$, and so we have:
a right triangle

THEOREM sum of angles in a triangle
The sum of the (interior) angles in any triangle is $\,180^{\circ}\,$.

A quick note about terminology:   you can't really ‘sum’ angles.
Angles are geometric figures, and we don't ‘add’ geometric figures.
When we talk about adding angles, it is understood that we're adding the measures of the angles.

Not only can we explore the interior (inside) angles of polygons,
but we can also explore exterior angles:

DEFINITION exterior angle in a polygon
An exterior angle of a polygon is an angle between one side of a polygon,
and the extension of an adjacent side.
an exterior angle in a polygon

Although two (equal) exterior angles can be drawn at each vertex of a polygon,
when we talk about summing the exterior angles of a polygon,
we agree to include only one exterior angle at each vertex.

For a triangle with interior angles $\,\alpha \,$, $\,\beta \,$, and $\,\gamma \,$, we have the following: $$ \cssId{s53}{(\alpha + \alpha_{\text{exterior}})} \cssId{s54}{+ (\beta + \beta_{\text{exterior}})} \cssId{s55}{+ (\gamma + \gamma_{\text{exterior}})} \cssId{s56}{= 3(180^{\circ})} $$
Regrouping gives: $$ \cssId{s58}{(\alpha +\beta +\gamma )} \cssId{s59}{+ (\alpha_{\text{exterior}} + \beta_{\text{exterior}} + \gamma_{\text{exterior}})} \cssId{s60}{= 3(180^{\circ})} $$
Since the sum of the (interior) angles of the triangle is $\,180^{\circ}\,$,
it follows immediately that: $$ \cssId{s63}{\alpha_{\text{exterior}} + \beta_{\text{exterior}} + \gamma_{\text{exterior}}} \cssId{s64}{= 2(180^{\circ})} \cssId{s65}{= 360^{\circ}} $$
summing exterior angle in a triangle

Thus we have the following:

THEOREM sum of exterior angles in a triangle
The sum of the exterior angles in any triangle is $\,360^{\circ}\,$.
Walking around the perimeter of a polygon—how much do you turn?

Indeed, you can think about the sum of the exterior angles in the following way.
Walk around the perimeter of the triangle above in a counter-clockwise direction.
(If we had labeled the other exterior angle at each vertex, you'd walk in the clockwise direction.)

At each vertex, you stop and pivot—you are turning an amount that is equal to an exterior angle!
When you reach your starting point, you are facing in exactly the same direction that you started,
so you have turned through a total of $\,360^{\circ}\,$—one complete revolution!
This is a nice way to think of why the exterior angles should sum to $\,360^{\circ}\,$.

(Try applying this thought process to the exterior angles in a more general polygon!
Can you make a conjecture?)


The interior angles of any polygon always sum to a constant value,
which depends only on the number of sides.

This is easy to see in many cases—take a polygon with $\,n\,$ sides,
and divide it into $\,n-2\,$ triangles, as shown at right.

Thus, the sum of the interior angles is $\,(n-2)(180^{\circ})\,$.

If the polygon ‘caves in’ then this method may break down,
as illustrated in the first sketch below.
The result is still true—it's just harder to prove!

summing interior angles in a polygon
summing interior angles in a non-convex polygon       summing interior angles in a non-convex polygon
THEOREM interior angles in polygons
Let $\,n\,$ be an integer, $\,n\ge 3\,$.

The sum of the interior angles in any $\,n$-gon is $\,(n-2)(180^{\circ})\,$.

Thus, in a regular polygon with $\,n\,$ sides, each interior angle has measure $\cssId{s101}{\displaystyle\,\frac{(n-2)(180^{\circ})}{n}}\,$.

You can explore interior and exterior angles of polygons at these excellent links:     interior angle exploration     exterior angle exploration
(Be patient; they may take a few minutes to load. They'll open in a new window.)

Master the ideas from this section
by practicing the exercise at the bottom of this page.

When you're done practicing, move on to:

On this exercise, you will not key in your answer.
However, you can check to see if your answer is correct.
1 2 3 4 5 6 7 8
9 10 11 12 13 14 15 16

(MAX is 16; there are 16 different problem types.)