The Exterior Angle Theorem

In the angle sum of a triangle post, we have discussed that the angle sum of a triangle is 180 degrees.  In the angle sum of a polygon post,  we also have discussed that  and that the angle sum of a polygon with n sides is 180(n-2). For example, a pentagon has 5 sides, so the sum of its interior angle is 180(5-2) = 180(3) = 540 degrees.

Figure 1 – The interior and exterior angles a triangle and a quadrilateral.

The angle sums that we have discussed in both blogs refer to the sum of the interior angles. What about the exterior angles?

The exterior angle is formed when we extend a side of a polygon. In the triangle above, \alpha is an exterior angle. The sum of the interior angle and the exterior angle adjacent to it is always 180  degrees (Why?).  Angles whose sum is 180 degrees are called supplementary angles.  If two angles are supplementary, we call them a linear pair.  For example, angles \alpha and a_1 are supplementary angles and at the same time a linear pair, so \alpha + a_1 = 180 degrees. Now this means, that \alpha = 180 - a_1. Therefore, if we want to compute the measure of an exterior angle adjacent to an interior angle, we can always subtract the measure of the interior angle from 180 as shown in Figure 1.

Observe the computation in the two diagrams.  If we let S_t be the angle sum of the exterior angles of a triangle, then S_t = (180 - a_1) + (180 - a_2) + (180 - a_3) = 540. Rearranging the terms, we have S_t = 540 - (a_1 + a_2 + a_3).  But a_1 + a_2 + a_3 is the sum of the interior angles of a triangle which is 180 degrees, so 540 - (a_1 + a_2 + a_3) = 540 - 180 = 360 degrees.

Now, try calculating for the sum of the exterior angles of the quadrilateral above. What is your answer?

To verify our hunch, we will try to compute for the sum of the exterior angles of a pentagon.

Let S_p be the sum of the exterior angles of the pentagon in Figure 2. Then

S_p =(180 - c_1)+ (180 - c_2) + (180 - c_3) +(180 - c_4) +(180 - c_5). Simplifying, we have S_p = 900 - (c_1 + c_2 + c_3 + c_4 + c_5). But according to the angle sum theorem for polygons, c_1 + c_2 + c_3 + c_4 + c_5 = 540. Therefore,900 - (c_1 + c_2 + c_3 + c_4 + c_5) = 900 - 540 = 360 degrees.

We have three polygons – triangle, quadrilateral, pentagon – whose angle sums of exterior angles are always 360 degrees. Now, is this true for all polygons?  Try to compute polygons up to 10 sides and see if the sum is 360 degrees.

Delving Deeper

We know that in a polygon, the number of exterior angles is equal to the number of interior angles.  Furthermore, we know that the angle sum of an interior angle and the exterior angle adjacent to each is always latex 180 degrees. If we have a polygon with 5 sides, then

interior angle sum + exterior angle sum = 180(5)

In general, this means that in a polygon with n sides

interior angle sum* + exterior angle sum = 180n

But the interior angle sum = 180(n – 2). So, substituting in the preceding equation, we have

180(n – 2) + exterior angle sum = 180n

which means that the exterior angle sum = 180n – 180(n – 2)  = 360 degrees. More formal proofs using these arguments are shown below.

Theorem: The sum of the measure of the exterior angles of a polygon with n sides is 360 degrees.

Proof 1:

Let a_1, a_2, \cdots a_n be measures of the interior angles of a polygon with n sides. Letb_1, b_2, \cdots b_n be measures of the exterior angles of the same polygon where all angle names with the same subscripts are adjacent angles from a_1 andb_1 all the way up through a_n and b_n .  We know that adjacent interior and exterior angles are supplementary angles, so this implies that their measures add up to 180 degrees. Hence,

(a1 + b1) + (a2 + b2) + … + (an + bn) = 180 + 180 + … +180 (n of them) = 180n

Regrouping the terms of the preceding equation, we have

(a1 + a2 + … + an) + (b1 + b2 + … + bn) = 180n

But the sum of the interior angles is a1 + a2 + … + an = 180(n – 2)

So,

180(n – 2) + (b1 + b2 + … + bn) = 180n

b1 + b2 + … + bn = 180n – 180(n – 2) = 360

Therefore, the sum of the exterior angles of any polygon is equal to 360 degrees.

Proof 2:

Let a1, a2, …, an be measures of the interior angles of the polygon with n sides. Since each adjacent interior and exterior angle is a linear pair, it follows that the measure of the exterior angles adjacent to them respectively are  180 – a1, 180 – a2, …, 180 – an.

If we let S, be the sum of the measure of the exterior angles, we have

S = (180 – a1) + (180 – a2) + (180 – a3) + … + (180 – an)

= (180 + 180 + 180 + … +180 (n of them)) – a – a2 – a3– … – an

S = 180n – (a1 + a2 + a3 + … + an)

But a1 + a2 + a3 + … + an is the sum of the measures of the interior angles of a polygon  with n sides which equals

180(n – 2), so, S = 180n – 180(n – 2) = 360, which is want we want to show.

Therefore, the sum of the exterior angles of any polygon is equal to 360 degrees.

Parallel Lines and Transversals

Given a triangle and a line segment, let us place the triangle such that one of its side and the line segment coincide. Next, we replicate the triangle and slide to the right hand side  such that two of their vertices meet as shown in Figure 1.

Figure 1 – Translated triangles have parallel corresponing sides.

The angles with the same color are basically the same angle, so we will call them corresponding angles. The sides which are parallel are also the same side, so we will call them corresponding sides.

What can you say about their corresponding angles? Their corresponding sides?

Figure 2 – The sum of the angles in point T is 180 degrees.

Next, we replicate one of the triangles, rotate it and place it between the two triangles as shown in Figure 2. Click here to rotate the triangles using GeoGebra. In the figure, we can observe and deduce the following:

  1. The measure of the three adjacent angles in T is half the circle and is therefore 180 degrees. This implies that the sum of the interior angles of triangle PQT is also 180 degrees, since they are the sum of the three adjacent angles in T.
  2. PQRT and QRST are parallelograms. (Explain why the opposite sides are parallel).
  3. The opposite angles of parallelogram are congruent, and their opposite sides are also congruent. (Explain why).
  4. We also observe that the sum of the consecutive angles of a parallelogram is 180 degrees. For example, the sum of angle PQR and angle QRT is 180 degrees.
  5. The interior angles of a parallelogram consists of 2 green angles, 2 yellow angles and 2 red angles, which means that the angle sum of the interior angles of a parallelogram is twice 180 or 360 degrees.
  6. PQRS is a trapezoid and the sum of its interior angles is also 360 degrees. (Explain why QR and PS are parallel).

If we extend QR and QT, then we will two parallel line segments and a transversal. In Figure 3, OR and PS are parallel lines a NU is a transversal. From Figure 1 and Figure 2, the yellow angles are corresponding angles, so RQT and PTQ are congruent.  We can also see that  the blue angle and yellow angle are supplementary (the sum of their measure is 180 degrees).

Figure 3 – NU is a transversal to the parallel lines OR and PS.

With the knowledge that the blue angle and the yellow angle are supplementary, we can deduce the following:

  1. Interior angles on the same side of a transversal of parallel lines (RQT and STQ) are supplementary.
  2. Angles OQN and OQT are supplementary so NQT must have the same measure as that of the yellow angle.
  3. Angles RQT and RQN are supplementary so RQN must have have the same measure as that of the blue angle.
  4. From 2 and 3, it follows that PTU is a blue angle and STU is a yellow angle.

With all the observations and deductions above, we came up with the following diagram. Note that angles symbols with the same color are congruent angles.

Figure 4 – Angle signs with the same color are congruent.

Pairs of these angles have special names  for distinction purposes. Most of the names are descriptive, so it is easy to determine other pairs. Some pairs of angles and their names are listed below.

1.) TQR and QTS are same side interior angles.

2.) NQR and STU are same side exterior angles.

3.) RQT and PTQ are alternate interior angles.

4.) STU and NQO are alternate exterior angles.

5.) NQO and QTP are corresponding angles.

Parallel lines and its relationship with its interval was mentioned in Euclid’s book The Elements. It is called the Fifth postulate or the Parallel Postulate. The parallel postulate states that

If a line segment intersects two straight lines forming two interior angles on the same side that sum to less than two right angles, then the two lines, if extended indefinitely, meet on that side on which the angles sum to less than two right angles.

All the relationships we have deduced above can be derived from this single statement.

I have also written why the angle sum of the interior angles of a triangle is 180 degrees. C

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