How to Solve Functional Problems Involving the Pythagorean Theorem

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  • 0:01 What is the…
  • 0:55 Why Does It Work?
  • 1:57 How Do We Use It?
  • 2:56 Finding the Hypoteneuse
  • 3:46 Finding a Shorter Side
  • 5:13 Lesson Summary
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Lesson Transcript
Instructor: Kevin Newton

Kevin has edited encyclopedias, taught history, and has an MA in Islamic law/finance. He has since founded his own financial advice firm, Newton Analytical.

One of the most useful aspects of studying triangles is the Pythagorean Theorem. In this lesson, we use it to find the unknown length of a side of a triangle, whether it is a shorter side or the hypotenuse.

What Is the Pythagorean Theorem?

Right triangles have long been some of the most interesting shapes that geometry has studied. In fact, as you progress further in math, you'll find that there is an entire subject dedicated to studying the ratios of their angles and lengths. But we're not there yet.

Instead, we're only interested in one ratio that makes use of all three sides of a triangle - the Pythagorean Theorem. Named for the ancient Greek philosopher Pythagoras, the theory is pretty simple and straightforward. a^2 + b^2 will equal c^2. Put another way, if you square the two shorter sides of a right triangle and add them together, you'll end up with the square of the hypotenuse.

In this lesson, we'll not only learn more about how it works, but we'll see how we can use it to find both the hypotenuse of a right triangle as well as the measure of one of the shorter sides.

Why Does It Work?

But first, why do we know that the Pythagorean Theorem even works? For that, we're going to need some cubes. 50, to be precise. Make three bigger pieces, one with 9 cubes as a square, another with 15 pieces as a square, and the last with 25 pieces as a square. Got them? Good. Now make a triangle out of them. You'll see that the one with the 25 pieces makes the hypotenuse of the triangle, while the other two make up the sides.

And what is the measure of these sides? 3, 4, and 5. Okay, so it's a useful trick, but how do we know for sure it works? For that, we'll need more cubes. If you want to keep playing with them, try these triangles for example: 6, 8, 10; 7, 14, 15; 33, 44, 55; or, if you're feeling really lucky: 37, 684, 685. Do you really want to count out all those blocks? I didn't think so, so just take my word on it!

How Do We Use It?

So now that we know it works, how do we use it? Simply put, just plug in the numbers that we know. For example, let's prove that massive triangle from earlier - the 37, 684, 685 triangle. Plug them into your formula of a^2 + b^2 = c^2. That means 37^2 + 684^2 = 685^2. In case you were curious, those numbers come out to being 1,369 + 467,856 = 469,225. This happens to add up to 469,225, which means that the triangle in question is definitely a right triangle.

So now that we've used it to prove that a triangle is a right triangle, let's work through a couple of examples that show us how to find either the hypotenuse or one of the sides.

Finding the Hypotenuse

Let's say you were trying to find the hypotenuse of a triangle with sides of 16 and 63. You know it's a right triangle, so how do you find the length of the hypotenuse? Let's plug in what we know. Remember a and b represent the shorter sides of the triangle, so that means a^2 + b^2 becomes 16^2 + 63^2 = c^2. That reduces to 256 + 3,969 = c^2, or 4,225 = c^2.

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