Until today we have dealt with internal angles, perhaps also with adjacent angles, but we have not talked about external angles. Don't worry, the topic of the exterior angle of a triangle is very easy to understand and its property can be very useful for solving exercises more quickly.

Shall we start?

The exterior angle of a triangle is the one that is found between the original side and the extension of the side.

Imagine someone draws a triangle and falls asleep as they are finishing it.

Without realizing it, they continue drawing one side a little more...

and Bam! An exterior angle is created.

The exterior angle is outside of the triangle and is found between the original side and the side they continued drawing while asleep (the continuation of the side).

**Observe:** An exterior angle is the one that is found between an original side of the triangle and the extension of the side and not between two extensions.

**Note:**

Whenever the angle is outside the triangle and is found between an original side of the triangle and the extension of another side of the triangle, it will be considered an exterior angle of the triangle.

Great! Now that we have understood what an exterior angle is and that we can recognize it from a distance, we can move on to the property of the exterior angle of a triangle.

Property of the exterior angle of a triangle

The exterior angle is equal to the sum of the two interior angles of the triangle that are not adjacent to it.

**Given that:**

$∢A=80$

$∢B=20$

How much does the exterior angle measure?

** Solution:**

** Let's denote the exterior angle with** **$α$****:**

According to the property of the exterior angle of the triangle, the exterior angle $α$ must be equal to the sum of the two interior angles of the triangle that are not adjacent to it.

That is, $∢A+∢B$

We already have these angles.

Therefore, all we have to do is add them up and find out the exterior angle:

$α=80+20$

$α=100$

Look, we could have found the value of the exterior angle in another way!

We know that the sum of the interior angles of a triangle is $180$.

Therefore, $∢ACB=180-20-80$

$∢ACB=80$

$∢ABC$ is the angle adjacent to $α$, the exterior angle of the triangle that we need to find out.

We also know that the sum of the adjacent angles is $180$.

Therefore we can determine that:

$∢80+α=80$

$α=100$

**Look, In certain cases you will not be explicitly asked for the value of the exterior angle.**

They might ask you, for example, about some interior angle of the triangle that you could figure out through the exterior angle.

Given the following triangle:

**Data:**

$∢A=90$

$α=110$

Find the value of $∢B$

**Solution:**

**We can solve the problem in two ways:**

The first is based on the Exterior Angle Theorem of a triangle and understand that $α$ is an exterior angle of the triangle and is equal to the sum of the two interior angles that are not adjacent to it. That is, $∢A+∢B$

Then,

** the equation would be:**

$110=90+∢B$

$∢B=20$

The second way to solve the problem is to remember that the sum of the adjacent angles equals $180$, then $∢ACB$ is equal to $70$.

$180-110=70$

Now, let's remember that the sum of the interior angles of a triangle is $180$

and we can find $∢B$

$∢B=180-90-70$

$∢B=20$

Notice that we have arrived at the same result, but solving through the property of the exterior angle of a triangle has been faster to reach it.

**Useful Information:**

The sum of the three exterior angles of a triangle equals $360$ degrees.

In conclusion, it is important and really worth knowing the property of the exterior angle of a triangle to solve problems easily and quickly, although in several cases you will be able to manage without this magnificent theorem.