The sum of two opposite interior angles equal the exterior angle
Here, we want to explain why the measure of the given angle equals the sum of the angles shown
Mathematically, we can get our answer by considering triangle BCD
What we can notice is that the two angles present in the sum lie inside the triangle while the other one lie outside the triangle
The angles that lie inside the triangle are known as interior angles while the angle that lie outside the triangle is known as the exterior angle
What we can also notice is that the two interior angles face each other. What this mean is that they are opposite each other
Thus, from here, we can conclude the following theorem;
sum of opposite interior angles equal the exterior angle
