Apply Pythagorean Theorem | Varsity Tutors

SECTION 1

Historical Context & Motivation

Thousands of years ago, people needed to build straight walls and measure land. They noticed something amazing about triangles that have a right angle (a 90° corner). The three sides of these triangles always follow a special pattern. This discovery became one of the most important ideas in all of math.

Ancient builders in Egypt and Babylon used this pattern to create perfect right angles for pyramids and fields. A Greek mathematician named Pythagoras is famous for writing down the rule and proving it works every time. That rule is called the Pythagorean Theorem.

~1800 BCE

Babylonian Clay Tablets

Babylonians carved number sets onto clay tablets that follow the Pythagorean pattern. This was over a thousand years before Pythagoras was born!

~500 BCE

Pythagoras Proves the Rule

The Greek mathematician Pythagoras gave the first known proof that the rule works for every right triangle, not just specific examples.

~300 BCE

Euclid's Elements

The famous math book "Elements" by Euclid included the theorem with detailed proofs. It became the standard geometry textbook for centuries.

Today

Used Everywhere

Engineers, architects, video game designers, and GPS systems all use the Pythagorean Theorem daily. It also appears on the HSPT!

So what exactly is this pattern? How do the three sides of a right triangle relate to each other? Let's find out.

SECTION 2

Core Principles & Definitions

Before using the Pythagorean Theorem, you need to know a few key words. A right triangle is any triangle that has exactly one 90° angle. The side across from the right angle is always the longest side. We call it the hypotenuse. The other two sides are called legs.

Right Triangle

A triangle with exactly one 90° angle (shown as a small square in the corner).

Hypotenuse

The longest side of a right triangle. It is always opposite (across from) the right angle.

Legs

The two shorter sides of a right triangle. They form the right angle where they meet.

The Theorem

In a right triangle, the sum of the squares of the two legs equals the square of the hypotenuse: a² + b² = c².

✦ KEY TAKEAWAY

Think of the Pythagorean Theorem like a recipe. If you know any two sides of a right triangle, you can always figure out the third side. It's like knowing two ingredients lets you figure out the missing one. The formula a² + b² = c² is your recipe card!

SECTION 3

Visual Explanation

The best way to understand the Pythagorean Theorem is to see it. Look at the diagram below. It shows a right triangle with squares built on each side. The area of the two smaller squares added together equals the area of the big square on the hypotenuse.

A 3-4-5 right triangle with squares on each side. The areas of the two leg squares (9 and 16) add up to the area of the hypotenuse square (25). This is the Pythagorean Theorem in action!

In the diagram, the triangle has legs of length 3 and 4, and a hypotenuse of length 5. When you square each side (multiply it by itself), you get 9 + 16 = 25. It works perfectly!

SECTION 4

The Formula & How to Use It

The Pythagorean Theorem is written as a simple equation. Let's look at it and learn what each letter means.

PYTHAGOREAN THEOREM

a² + b² = c²

a = one leg of the right triangle, b = the other leg, c = the hypotenuse (always the longest side, across from the right angle)

You can use this formula in two ways. If you know both legs, you can find the hypotenuse. If you know the hypotenuse and one leg, you can find the missing leg.

FINDING THE HYPOTENUSE

c = √(a² + b²)

Square both legs, add them together, then take the square root to find c.

FINDING A MISSING LEG

a = √(c² − b²)

Square the hypotenuse, subtract the known leg squared, then take the square root to find the missing leg.

💡 Remember!

The letter c always stands for the hypotenuse. It doesn't matter which leg you call a and which you call b — the two legs are interchangeable.

SECTION 5

Common Pythagorean Triples

Some right triangles have sides that are all whole numbers. These special sets are called Pythagorean triples. Memorizing a few of them can save you time on the HSPT because you'll recognize the answer right away!

Common Pythagorean Triples to Memorize

Leg a	Leg b	Hypotenuse c	Check: a² + b² = c²
3	4	5	9 + 16 = 25 ✓
5	12	13	25 + 144 = 169 ✓
8	15	17	64 + 225 = 289 ✓
6	8	10	36 + 64 = 100 ✓
7	24	25	49 + 576 = 625 ✓

Three commonly tested Pythagorean triples: 3-4-5, 5-12-13, and 8-15-17. Each triangle shows its leg and hypotenuse values. Notice the hypotenuse (in pink) is always the longest side.

⭐ HSPT Tip

The triple 6-8-10 is just the 3-4-5 triple with every number doubled! You can multiply any triple by the same number to get a new one. So 3-4-5 also gives you 9-12-15 (multiplied by 3) and 12-16-20 (multiplied by 4).

SECTION 6

Worked Example

Let's walk through a full problem step by step. This is the type of problem you might see on the HSPT.

Finding the Hypotenuse

Step 1 — Read the ProblemA right triangle has legs of length 6 and 8. What is the length of the hypotenuse?

Step 2 — Write the FormulaStart with the Pythagorean Theorem: a² + b² = c². We know the two legs, so we need to find c (the hypotenuse).

Step 3 — Plug In the NumbersSubstitute a = 6 and b = 8 into the formula:

6² + 8² = c²

Step 4 — Square Each LegCalculate 6² = 6 × 6 = 36, and 8² = 8 × 8 = 64.

36 + 64 = c²

Step 5 — Add the SquaresAdd the two results together: 36 + 64 = 100.

100 = c²

Step 6 — Take the Square RootTo undo the square, take the square root of both sides. √100 = 10.

c = 10

Finding a Missing Leg

Step 1 — Read the ProblemA right triangle has a hypotenuse of 13 and one leg of 5. What is the other leg?

Step 2 — Write the Formulaa² + b² = c². This time we know c = 13 and one leg b = 5. We need to find a.

Step 3 — Plug In and RearrangeSubstitute: a² + 5² = 13². Then rearrange: a² = 13² − 5².

a² = 169 − 25

Step 4 — Subtract169 − 25 = 144.

a² = 144

Step 5 — Take the Square Root√144 = 12.

a = 12

SECTION 7

Common Mistakes & HSPT Tips

The Pythagorean Theorem is straightforward, but there are a few traps students fall into. Knowing these ahead of time will help you avoid losing easy points on the HSPT.

Avoid These Common Errors

Common Mistake	Why It's Wrong	What to Do Instead
Putting the hypotenuse in for a or b	The hypotenuse must always be c. Using it as a leg gives a wrong answer.	Always identify the longest side first and label it c.
Adding the sides instead of squaring them	The formula is a² + b² = c², not a + b = c.	Square each side first, then add or subtract.
Forgetting to take the square root	After adding the squares, you have c², not c. You need one more step.	Always take √ at the end to get the actual side length.
Using it on non-right triangles	The theorem only works for right triangles. No right angle = can't use it.	Check for a right angle (90°) before applying the formula.

✦ KEY TAKEAWAY

Think of the hypotenuse like the team captain — it always gets the special spot in the formula (c). The two legs are teammates that work together (a² + b²). Don't mix up who's the captain!

SECTION 8

Beyond the Basics

Once you master the basic formula, you can use the Pythagorean Theorem in many cool ways. On the HSPT, you might see it hidden inside word problems about distance, ladders, or diagonal measurements.

How the Pythagorean Theorem Connects to Future Math

What You Know Now	What Comes Next in High School
Finding sides of right triangles using a² + b² = c²	The Distance Formula: finding the distance between any two points on a graph
Working with whole-number Pythagorean triples	Trigonometry: sine, cosine, and tangent ratios for any angle
2D right triangles	3D distance: extending the theorem to find diagonals inside boxes and cubes

Here's a preview: the Distance Formula you'll learn in high school is actually just the Pythagorean Theorem in disguise! When you find the distance between two points on a coordinate plane, you're really finding the hypotenuse of a right triangle. So mastering this theorem now gives you a head start.

🌍 Real-World Connection

Your phone's GPS uses a version of the Pythagorean Theorem to calculate distances. When a map app tells you a place is "0.5 miles away," it's using this same math behind the scenes!

SECTION 9

Practice Problems

Try these five problems on your own. They start easy and get harder. Work through each one before checking the answer!

PROBLEM 1 — CONCEPTUAL

In a right triangle, which side is the hypotenuse: the side across from the right angle, or one of the sides that forms the right angle?

PROBLEM 2 — BASIC CALCULATION

A right triangle has legs of length 3 and 4. What is the length of the hypotenuse?

PROBLEM 3 — INTERMEDIATE

A right triangle has a hypotenuse of 17 and one leg of 8. What is the length of the other leg?

PROBLEM 4 — APPLIED

A 10-foot ladder leans against a wall. The bottom of the ladder is 6 feet from the base of the wall. How high up the wall does the ladder reach?

PROBLEM 5 — CRITICAL THINKING

A rectangular field is 9 meters long and 12 meters wide. If you walk diagonally from one corner to the opposite corner, how far do you walk? Then explain: why does a diagonal of a rectangle create a right triangle?

SUMMARY

Lesson Summary

The Pythagorean Theorem says that in any right triangle, the sum of the squares of the two legs equals the square of the hypotenuse: a² + b² = c². The hypotenuse (c) is always the longest side and sits across from the 90° angle. You can use the formula to find any missing side when you know the other two.

Memorize common Pythagorean triples like 3-4-5, 5-12-13, and 8-15-17 to save time on the HSPT. Remember to always identify the hypotenuse first, square before adding, and take the square root at the end. Look for hidden right triangles in word problems about ladders, diagonals, and distances!