Triangle inequality theorem proof

Before you understand the triangle inequality theorem proof, you need to review the triangle inequality theorem and understand the shortest distance theorem.

Shortest distance theorem:

The shortest distance from a point p to a line s is the line perpendicular to s and passing through p.

This is illustrated below. As you can see the shortest distance is segment PR and this is shown in blue.

Any other segments such as segment PF or segment PO ( shown in red) is longer.

Shortest distance theorem
You have to understand this theorem before trying to comprehend the proof about triangle inequality theorem.

Suppose you have a triangle ABC. The proof must have the following 3 parts.

BA + AC > BC

BA + BC > AC

BC + AC > BA

In this lesson, we will prove that BA + AC > BC and BA + BC > AC. 

It will be up to you to prove that BC + AC > BA

Now, here is the triangle inequality theorem proof

Draw any triangle ABC and the line perpendicular to BC passing through vertex A. (This is shown in blue)

triangle inequality theorem proof


Now prove that BA + AC > BC.

BE is the shortest distance from vertex B to AE.

This means that BA > BE.

By the same token,

CE is the shortest distance from C to AE.

This means that AC > CE.

Let us put it all together:

BA > BE and

AC > CE

Add the left side and add the right side of the inequalities. This gives:

BA + AC > BE + CE

Now, notice that BE + CE = BC

Therefore, BA + AC > BC

Now, starting with the same triangle, draw the line perpendicular to AC passing through vertex B. (This is shown in blue)


triangle inequality theorem proof


Prove that BA + BC > AC

AE is the shortest distance from vertex A to BE

This means that BA > AE

By the same token,

CE is the shortest distance from C to BE

This means that BC > CE

Let us put it all together:

BA > AE and

BC > CE

Add the left side and add the right side of the inequalities. This gives:

BA + BC > AE + CE

Now, notice that AE + CE = AC

Therefore, BA + BC > AC

Now, here is your exercise: Try to prove that AC + BC > AB

Recent Articles

  1. How To Find The Factors Of 20: A Simple Way

    Sep 17, 23 09:46 AM

    Positive factors of 20
    There are many ways to find the factors of 20. A simple way is to...

    Read More

  2. The SAT Math Test: How To Be Prepared To Face It And Survive

    Jun 09, 23 12:04 PM

    SAT math
    The SAT Math section is known for being difficult. But it doesn’t have to be. Learn how to be prepared and complete the section with confidence here.

    Read More

Tough algebra word problems

100 Tough Algebra Word Problems.

If you can solve these problems with no help, you must be a genius!

Math quizzes

 Recommended

Math vocabulary quizzes