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4.1: Triangle Sum Theorem

Difficulty Level: At Grade Created by: CK-12
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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

CK-12 Foundation: Chapter4TriangleSumTheoremA

James Sousa: Animation of the Sum of the Interior Angles of a Triangle

James Sousa: Proving the Triangle Sum Theorem


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 \begin{align*}\triangle\end{align*}. This triangle can be labeled \begin{align*}\triangle ABC, \triangle ACB, \triangle BCA, \triangle BAC, \triangle CBA\end{align*} or \begin{align*}\triangle CAB\end{align*}. Order does not matter. The angles in any polygon are measured in degrees. Each polygon has a different sum of degrees, depending on the number of angles in the polygon. How many degrees are in a triangle?

Investigation: Triangle Tear-Up

Tools Needed: paper, ruler, pencil, colored pencils

  1. 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, \begin{align*}\angle 1, \angle 2,\end{align*} and \begin{align*}\angle 3\end{align*}.
  2. Tear off the three colored angles, so you have three separate angles.
  3. 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 \begin{align*}180^\circ\end{align*} because the three angles fit together to form a straight line. Recall that a line is also a straight angle and all straight angles are \begin{align*}180^\circ\end{align*}.

The Triangle Sum Theorem states that the interior angles of a triangle add up to \begin{align*}180^\circ\end{align*}. The above investigation is one way to show that the angles in a triangle add up to \begin{align*}180^\circ\end{align*}. However, it is not a two-column proof. Here we will prove the Triangle Sum Theorem.

Given: \begin{align*}\triangle ABC\end{align*} with \begin{align*}\overleftrightarrow{AD} \ || \ \overline{BC}\end{align*}

Prove: \begin{align*}m \angle 1+m \angle 2+m \angle 3=180^\circ\end{align*}

Statement Reason
1. \begin{align*}\triangle ABC\end{align*} above with \begin{align*}\overleftrightarrow{AD} \ || \ \overline{BC}\end{align*} Given
2. \begin{align*}\angle 1 \cong \angle 4, \angle 2 \cong \angle 5\end{align*} Alternate Interior Angles Theorem
3. \begin{align*}m \angle 1 = m \angle 4, m \angle 2 = m \angle 5\end{align*} \begin{align*}\cong\end{align*} angles have = measures
4. \begin{align*}m \angle 4 + m \angle CAD = 180^\circ\end{align*} Linear Pair Postulate
5. \begin{align*}m \angle 3 + m \angle 5 = m \angle CAD\end{align*} Angle Addition Postulate
6. \begin{align*}m \angle 4 + m \angle 3 + m \angle 5 = 180^\circ\end{align*} Substitution PoE
7. \begin{align*}m \angle 1 + m \angle 3 + m \angle 2 = 180^\circ\end{align*} 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 \begin{align*}60^\circ\end{align*}.

Theorem #2: The acute angles in a right triangle are always complementary.

Example A

What is the \begin{align*}m \angle T\end{align*}?

From the Triangle Sum Theorem, we know that the three angles add up to \begin{align*}180^\circ\end{align*}. Set up an equation to solve for \begin{align*}\angle T\end{align*}.

\begin{align*}m \angle M + m \angle A + m \angle T &= 180^\circ\\ 82^\circ + 27^\circ + m \angle T &= 180^\circ\\ 109^\circ + m \angle T &= 180^\circ\\ m \angle T &= 71^\circ\end{align*}

Example B

Show why Theorem #1 is true.

\begin{align*}\triangle ABC\end{align*} above is an example of an equiangular triangle, where all three angles are equal. Write an equation.

\begin{align*}m \angle A+m \angle B+m \angle C &= 180^\circ\\ m \angle A+m \angle A+m \angle A &= 180^\circ\\ 3m \angle A &= 180^\circ\\ m \angle A &= 60^\circ\end{align*}

If \begin{align*}m \angle A = 60^\circ\end{align*}, then \begin{align*}m \angle B = 60^\circ\end{align*} and \begin{align*}m \angle C = 60^\circ\end{align*}.

Example C

Use the picture below to show why Theorem #2 is true.

\begin{align*}m \angle O = 41^\circ\end{align*} and \begin{align*}m \angle G = 90^\circ\end{align*} because it is a right angle.

\begin{align*}m \angle D+m \angle O+m \angle G &= 180^\circ\\ m \angle D+41^\circ+90^\circ &= 180^\circ\\ m \angle D+41^\circ &= 90^\circ\\ m \angle D &= 49^\circ\end{align*}

Notice that \begin{align*}m \angle D + m \angle O = 90^\circ\end{align*} because \begin{align*}\angle G\end{align*} is a right angle.

Watch this video for help with the Examples above.

CK-12 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 \begin{align*}180^\circ\end{align*}. However, because your measures are estimates using a protractor, they might not exactly add up.

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.


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 \begin{align*}90^\circ\end{align*}.

Obtuse: One angle is greater than \begin{align*}90^\circ\end{align*}.

Equiangular: All three angles are congruent.

Guided Practice

1. Determine \begin{align*}m\angle{1}\end{align*} in this triangle:

2. Two interior angles of a triangle measure \begin{align*}50^\circ\end{align*} and \begin{align*}70^\circ\end{align*}. What is the third interior angle of the triangle?

3. Find the value of \begin{align*}x\end{align*} and the measure of each angle.


1. \begin{align*}72^\circ + 65^\circ +m\angle{1} = 180^\circ \end{align*}.

Solve this equation and you find that \begin{align*}m\angle{1}=43^\circ\end{align*}.

2. \begin{align*}50^\circ + 70^\circ + x = 180^\circ \end{align*}.

Solve this equation and you find that the third angle is \begin{align*}60^\circ\end{align*}.

3. All the angles add up to \begin{align*}180^\circ\end{align*}.

\begin{align*}(8x-1)^\circ + (3x+9)^\circ+(3x+4)^\circ&=180^\circ\\ (14x+12)^\circ&=180^\circ\\ 14x = 168\\ x =12\end{align*}

Substitute in 12 for \begin{align*}x\end{align*} to find each angle.

\begin{align*}[3(12) + 9]^\circ = 45^\circ && [3(12) + 4]^\circ = 40^\circ && [8(12) - 1]^\circ = 95^\circ\end{align*}


Determine \begin{align*}m\angle{1}\end{align*} in each triangle.








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.







Triangle Sum Theorem

Triangle Sum Theorem

The Triangle Sum Theorem states that the three interior angles of any triangle add up to 180 degrees.

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Difficulty Level:

At Grade


Date Created:

Jul 17, 2012

Last Modified:

Feb 26, 2015
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