4.1: Triangle Sum Theorem
What if you wanted to classify the Bermuda Triangle by its sides and angles? You are probably familiar with the myth of this triangle; how several ships and planes passed through and mysteriously disappeared.
The measurements of the sides of the triangle are in the image. What type of triangle is this? Using a protractor, find the measure of each angle in the Bermuda Triangle. What do they add up to? Do you think the three angles in this image are the same as the three angles in the actual Bermuda triangle? Why or why not? After completing this Concept, you'll be able to determine how the three angles in any triangle are related in order to help you answer these questions.
Watch This
CK12 Foundation: Chapter4TriangleSumTheoremA
James Sousa: Animation of the Sum of the Interior Angles of a Triangle
James Sousa: Proving the Triangle Sum Theorem
Guidance
In polygons, interior angles are the angles inside of a closed figure with straight sides. The vertex is the point where the sides of a polygon meet.
Triangles have three interior angles, three vertices and three sides. A triangle is labeled by its vertices with a
Investigation: Triangle TearUp
Tools Needed: paper, ruler, pencil, colored pencils
 Draw a triangle on a piece of paper. Try to make all three angles different sizes. Color the three interior angles three different colors and label each one,
∠1,∠2, and∠3 .  Tear off the three colored angles, so you have three separate angles.
 Attempt to line up the angles so their points all match up. What happens? What measure do the three angles add up to?
This investigation shows us that the sum of the angles in a triangle is
The Triangle Sum Theorem states that the interior angles of a triangle add up to
Given:
Prove:
Statement  Reason 

1. 
Given 
2. 
Alternate Interior Angles Theorem 
3. 

4. 
Linear Pair Postulate 
5. 
Angle Addition Postulate 
6. 
Substitution PoE 
7. 
Substitution PoE 
There are two theorems that we can prove as a result of the Triangle Sum Theorem and our knowledge of triangles.
Theorem #1: Each angle in an equiangular triangle measures
Theorem #2: The acute angles in a right triangle are always complementary.
Example A
What is the
From the Triangle Sum Theorem, we know that the three angles add up to
Example B
Show why Theorem #1 is true.
If
Example C
Use the picture below to show why Theorem #2 is true.
Notice that
Watch this video for help with the Examples above.
CK12 Foundation: Chapter4TriangleSumTheoremB
Concept Problem Revisited
The Bermuda Triangle is an acute scalene triangle. The angle measures are in the picture to the right. Your measured angles should be within a degree or two of these measures. The angles should add up to
The angle measures in the picture are the actual measures, based off of the distances given, however, your measured angles might be off because the drawing is not to scale.
Vocabulary
A triangle is a three sided shape. All triangles have three interior angles, which are the inside angles connecting the sides of the triangle. The vertex is the point where the sides of a polygon meet. Special types of triangles are listed below:
Scalene: All three sides are different lengths.
Isosceles: At least two sides are congruent.
Equilateral: All three sides are congruent.
Right: One right angle.
Acute: All three angles are less than
Obtuse: One angle is greater than
Equiangular: All three angles are congruent.
Guided Practice
1. Determine
2. Two interior angles of a triangle measure
3. Find the value of
Answers:
1.
Solve this equation and you find that
2.
Solve this equation and you find that the third angle is
3. All the angles add up to
Substitute in 12 for
\begin{align*}[3(12) + 9]^\circ = 45^\circ && [3(12) + 4]^\circ = 40^\circ && [8(12)  1]^\circ = 95^\circ\end{align*}
Practice
Determine \begin{align*}m\angle{1}\end{align*} in each triangle.
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8. Two interior angles of a triangle measure \begin{align*}32^\circ\end{align*} and \begin{align*}64^\circ\end{align*}. What is the third interior angle of the triangle?
9. Two interior angles of a triangle measure \begin{align*}111^\circ\end{align*} and \begin{align*}12^\circ\end{align*}. What is the third interior angle of the triangle?
10. Two interior angles of a triangle measure \begin{align*}2^\circ\end{align*} and \begin{align*}157^\circ\end{align*}. What is the third interior angle of the triangle?
Find the value of \begin{align*}x\end{align*} and the measure of each angle.
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Image Attributions
Here you'll learn that the sum of the angles in any triangle is the same, due to the Triangle Sum Theorem.