Answer:
See explanation below.
Step-by-step explanation:
There is a theorem about the sum of the measures of the angles of a triangle.
In this case, with the interior angle measures called a, b, and c, the theorem states:
Theorem:
The sum of the measures of the interior angles of a triangle is 180°.
a + b + c = 180°
Now we will prove the theorem and answer why a + b = d.
Look at angles c and d. They form a linear pair. Angles that form a linear pair are supplementary angles. By the definition of supplementary angles, the sum of the measures of angles c and d is 180°.
We have
c + d = 180° Eq. 1
By the theorem above, we have
a + b + c = 180° Eq. 2
Let's subtract Eq. 1 from Eq. 2:
a + b + c = 180°
- c + d = 180°
------------------------------
a + b - d = 0
a + b - d = 0
Add d to both sides.
a + b = d
That is a proof of the Exterior Angle Theorem.