The HA (Hypotenuse Angle) Theorem: Proof, Explanation, & Examples

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  • 0:05 The HA Theorem
  • 1:04 Proving the Theorem
  • 2:24 Practice Proof #1
  • 3:53 Practice Proof #2
  • 5:10 Lesson Summary
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Lesson Transcript
Instructor: Jeff Calareso

Jeff teaches high school English, math and other subjects. He has a master's degree in writing and literature.

In this lesson, we'll learn about the hypotenuse angle theorem. With this theorem, we can prove two right triangles are congruent with just congruent hypotenuses and acute angles.

The HA Theorem

If you're a triangle, finding out that you're congruent to another triangle is a big deal. Imagine finding out one day that you have a twin that you didn't know about. How amazing would that be? It's like having a spare 'you' suddenly enter your life.

In geometry, we try to find triangle twins in any way we can. There are all kinds of methods, like side-side-side, angle-side-angle, side-angle-side and more. In the real world, it doesn't work that way. You can't just compare legs with a stranger to test for congruency.

With two right triangles, we already know that they have something in common - those right angles. So, it's like they're at least cousins. And we can prove they're congruent with the hypotenuse angle theorem.

The hypotenuse angle theorem, also known as the HA theorem, states that 'if the hypotenuse and an acute angle of one right triangle are congruent to the hypotenuse and an acute angle of another right triangle, then the two triangles are congruent.'

Proving the Theorem

How can we verify congruency with just a hypotenuse and an acute angle? It's like saying two people are twins because they have the same height and hair color. Let's look at a couple of triangles.

Here's triangle ABC:

Example triangle ABC
example triangle

Now along comes triangle XYZ:

Example triangle XYZ
example triangle

Could they be twins? They're both right triangles. Angles B and Y are each 90 degrees. We're told that AC is congruent to XZ. So, that's one hypotenuse that's congruent to the other. And we're told that angle A is congruent to angle X. That's good, but it's not like a DNA test.

Or is it? In triangle ABC, what's the sum of the interior angles? 180. What about with triangle XYZ? It's also 180. So, if two angles are congruent, like A and X, and another two angles are congruent, like B and Y, then the other angles, C and Z, must also be congruent.

So, now we have angle A, side AC and angle C congruent to angle X, side XZ and angle Z. And that's angle-side-angle, or ASA. That means that the HA theorem is really just a simplification or variation of the ASA postulate that works with right triangles.

Practice Proof #1

Let's try to find some twins with some proofs. You know, you're not twins without proof. Here are two triangles:

Two example triangles
example triangles

They're very close. Together, they look kinda like a kite, don't they? Maybe they like to fly kites together. But are they just really good friends, or are they twins?

We're given that angles R and S are right angles. And we're also given that angle SQT is congruent to angle RQT. Let's say we want to determine if RT is congruent to ST.

Let's start our proof by collecting DNA samples from each triangle. Wait, what? Two-dimensional polygons don't have DNA? Oh. Then I guess we'll need to do an ordinary proof. Okay, first, we know that angles R and S are right angles. We're given that. That means that triangles QST and QRT are right triangles. That's the definition of a right triangle.

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