In triangle ABC with additional lines PQ and MN, corresponding angles
and
. Supplementary angles
, proving the triangle angle sum theorem.
To prove that the sum of the interior angles of a triangle is equal to 180 degrees, we can consider the triangle ABC and the additional lines PQ and MN. Let's denote the angles as follows:
-

-

-

-

Now, let's analyze the pairs of angles to see which ones must be congruent and which ones are supplementary:
1. Congruent Angles:
-
must be congruent. This is because they are corresponding angles between parallel lines PQ and MN, cut by transversal line BC.
-
must be congruent. Similar to the first case, these are corresponding angles between parallel lines PQ and MN, cut by transversal line BA.
So, we can say:

2. Supplementary Angles:
- The pair
must be supplementary. This is because they form a linear pair within the triangle ABC.
So, we can say:

Now, let's use these relationships to prove that the sum of the interior angles of triangle ABC is equal to 180 degrees:
![\[ \angle PBA + \angle BAM + \angle QBC = \angle PBA + \angle BAM + \angle PBA \cong \angle PBA + \angle BAM + \angle BAM = 180^\circ. \]](https://img.qammunity.org/2024/formulas/mathematics/high-school/p4si6juukq1xy8lf2qb9j3lune4ekpjq5n.png)
Therefore, the sum of the interior angles of triangle ABC is equal to 180 degrees, and we used the congruent and supplementary relationships between the angles to establish this result.