Skip to main content
Search IntMath
Close

450+ Math Lessons written by Math Professors and Teachers

5 Million+ Students Helped Each Year

1200+ Articles Written by Math Educators and Enthusiasts

Simplifying and Teaching Math for Over 23 Years

Tips, tricks, lessons, and tutoring to help reduce test anxiety and move to the top of the class.

The Angle Sum Property in Geometry

In geometry, the angle sum property states that the sum of the angles in a triangle is 180 degrees. This property is also known as the Triangle Inequality Theorem. The theorem states that the sum of the lengths of any two sides of a triangle must be greater than or equal to the length of the third side. 

 

The angle sum property is a result of the fact that a straight line creates a 180 degree angle. When you draw a line from one vertex (corner) of a triangle to another vertex and then to the third vertex, you create two straight lines and, therefore, two 180 degree angles. This means that the sum of all three angles in a triangle must be 180 degrees. 

 

How to Prove the Angle Sum Property 

There are two ways that you can prove the angle sum property. The first way uses algebra and basic properties of angles. The second way uses trigonometry. We'll go over both methods so that you can see how they work. 

 

Method 1: Algebraic Proof 

Step 1: Label the angles in your triangle as follows: 

 

Angle A + Angle B + Angle C = Angle X 

 

Step 2: Use the properties of angles to rewrite Angle X in terms of known values. Remember that when two angles are adjacent (share a side), their measurements add up to 90 degrees. You can also label Angle X as 2 times Angle Y (since it's twice the size). This gives us: 

 

Angle A + Angle B + Angle C = 2(Angle Y) 

 

Step 3: Substitute what you know about right triangles for Angle Y. A right triangle is a type of triangle where one angle is 90 degrees. This means that the other two angles must add up to 90 degrees as well. So we can write: 

 

Angle A + Angle B + Angle C = 2(90) 

 

Step 4: Solve for Angle C. This gives us: 

 

Angle C = 180 - (Angle A + Angle B) 

 

We've now proven that the sum of the angles in any triangle is 180 degrees! 

 

Method 2: Trigonometric Proof  

Step 1: Pick any angle in your triangle and label it Opposite Side A. Then use basic trigonometry to find its measurement in terms of known values. Trigonometry is a branch of mathematics that deals with triangles and measuring angles—it's what allows us to find things like "the cosine of an angle." We'll use basic trigonometry formulas to solve for our unknown value, which we'll call Opposite Side A. In this case, we'll use SohCahToa, which states that: 

 

 Sin(angle) = Opposite Side / Hypotenuse 

 Cos(angle) = Adjacent Side / Hypotenuse  

 Tan(angle) = Opposite Side / Adjacent Side    

 

Step 2: Substitute what you know about right triangles for Sin(angle), Cos(angle), and Tan(angle). Remember that in a right triangle, one angle will always be 90 degrees—this means that we can use some basic trigonometry ratios to solve for our unknown value, which is still Opposite Side A . In this case, we'll use SohCahToa, which states that: 

 

 Sin(90) = Opposite Side / Hypotenuse     

Cos(90) = Adjacent Side / Hypotenuse  

Tan(90) = Opposite Side / Adjacent Side  

 

Step 3: Solve for Opposite Side A . This gives us:  

 

 Opposite Side A = 1 *Hypotenuse  Since Sin(90)=1 , we can say that Sin(90)=1 *Hypotenuse . Therefore, Opposite Side A must equal 1 *Hypotenuse . Thus, we have proven that all three sides of a right triangle are connected by this equation! 

 

 Now let's take it one step further and prove that this equation works for all types of triangles—not just right triangles...  

 

Step 4: Assume that your triangle is not a right triangle but instead has sides AB , BC , and AC . Extend side AC past point C until it intersects side AB at some point D , as shown below:  Now we have created two new triangles, Triangle ABC and Triangle ADC . Notice how Triangle ADC contains one 90 degree angle—this makes it a right triangle! Since we already know that all three sides of a right triangle are connected by this equation, we can say that AD=1 *BC . But wait—what does this tell us about Triangle ABC ? Well, since AD=1 *BC , then we can also say that AB=1 *DC ! Thus, this equation proves true for all types of triangles—not just right triangles! And there you have it—two different ways to prove the angle sum property!

 

FAQ

How do you prove the angle sum property?

There are two ways to prove the angle sum property: algebraically or trigonometrically. To prove it algebraically, label the angles in your triangle and use the properties of angles to rewrite Angle X in terms of known values. Then substitute what you know about right triangles for Angle Y. This will give you an equation that you can solve for Angle C. To prove it trigonometrically, use basic trigonometry to find the measurement of one angle in terms of known values. Then substitute what you know about right triangles for Sin(angle), Cos(angle), and Tan(angle). This will give you an equation that you can solve for Opposite Side A.

 

How do you prove the sum of the angles of a triangle?

There are two ways to prove the angle sum property: algebraically or trigonometrically. To prove it algebraically, label the angles in your triangle and use the properties of angles to rewrite Angle X in terms of known values. Then substitute what you know about right triangles for Angle Y. This will give you an equation that you can solve for Angle C. To prove it trigonometrically, use basic trigonometry to find the measurement of one angle in terms of known values. Then substitute what you know about right triangles for Sin(angle), Cos(angle), and Tan(angle). This will give you an equation that you can solve for Opposite Side A.

 

How do you prove a sum?

There are two ways to prove the angle sum property: algebraically or trigonometrically. To prove it algebraically, label the angles in your triangle and use the properties of angles to rewrite Angle X in terms of known values. Then substitute what you know about right triangles for Angle Y. This will give you an equation that you can solve for Angle C. To prove it trigonometrically, use basic trigonometry to find the measurement of one angle in terms of known values. Then substitute what you know about right triangles for Sin(angle), Cos(angle), and Tan(angle). This will give you an equation that you can solve for Opposite Side A about right triangles for Angle.

24x7 Tutor Chat

Tips, tricks, lessons, and tutoring to help reduce test anxiety and move to the top of the class.