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CK-12 FOUNDATION

CK-12 Geometry - Pythagorean

Theorem

Say Thanks to the Authors

Click http://www.ck12.org/saythanks

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Swenson

To access a customizable version of this book, as well as other interactive content, visit www.ck12.org

CK-12 Foundation is a non-proﬁt organization with a mission to reduce the cost of textbook materials for the K-12 market both in the U.S. and worldwide. Using an open-content, web-based collaborative

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by this reference.

Complete terms can be found at http://www.ck12.org/terms.

Printed: August 2, 2011

Author

Laura Swenson

Editor

Joy Sheng

i

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Contents

1 History of the Pythagorean

Theorem

1

2 Applying the Pythagorean

Theorem

7

3 Proving the Pythagorean

Theorem

19

4 Exercises

27

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ii

Chapter 1

History of the Pythagorean

Theorem

Pythagoras and the Pythagoreans

More than 2,500 years ago, around 530 BCE, a man by the name of Pythagoras founded a school in

modern southeast Italy. Members of the school, which was actually more of a brotherhood, were bound

by a pledge of allegiance to their master Pythagoras and took an oath of silence to not divulge secret

discoveries. Pythagoreans shared a common belief in the supremacy of numbers, using them to describe

and understand everything from music to the physical universe. Studying a wide range of intellectual

disciplines, Pythagoreans made a multitude of discoveries, many of which were attributed to Pythagoras

himself. No records remain of the actual discoverer, so the identity of the true discoverer may never be

known. Perhaps the most famous of the Pythagoreans’ contributions to knowledge is proving what has

come to be known as the Pythagorean Theorem.

Pythagorean Theorem

The Pythagorean Theorem allows you to ﬁnd the lengths of the sides of a right triangle, which is a

triangle with one 90◦ angle (known as the right angle). An example of a right triangle is depicted below.

A right triangle is composed of three sides: two legs, which are labeled in the diagram as leg1 and leg2 ,

and a hypotenuse, which is the side opposite to the right angle. The hypotenuse is always the longest of

the three sides. Typically, we denote the right angle with a small square, as shown above, but this is not

required.

The Pythagorean Theorem states that the length of the hypotenuse squared equals the sum of the squares

of the two legs. This is written mathematically as:

1

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(leg1 )2 + (leg2 )2 = (hypotenuse)2

To verify this statement, ﬁrst explicitly expressed by Pythagoreans so many years ago, let’s look at an

example.

Example 1

Consider the right triangle below. Does the Pythagorean Theorem hold for this triangle?

Solution

As labeled, this right triangle has sides with lengths 3, 4, and 5. The side with length 5, the longest side,

is the hypotenuse because it is opposite to the right angle. Let’s say the side of length 4 is leg1 and the

side of length 3 is leg2 .

Recall that the Pythagorean Theorem states:

(leg1 )2 + (leg2 )2 = (hypothenuse)2

If we plug the values for the side lengths of this right triangle into the mathematical expression of the

Pythagorean Theorem, we can verify that the theorem holds:

(4)2 + (3)2 = (5)2

16 + 9 = 25

25 = 25

Although it is clear that the theorem holds for this speciﬁc triangle, we have not yet proved that the

theorem will hold for all right triangles. A simple proof, however, will demonstrate that the Pythagorean

Theorem is universally valid.

Proof Based on Similar Triangles

The diagram below depicts a large right triangle (triangle ABC) with an altitude (labeled h) drawn from

one of its vertices. An altitude is a line drawn from a vertex to the side opposite it, intersecting the side

perpendicularly and forming a 90◦ angle.

In this example, the altitude hits side AB at point D and creates two smaller right triangles within the

larger right triangle. In this case, triangle ABC is similar to triangles CBD and ACD. When a triangle is

similar to another triangle, corresponding sides are proportional in lengths and corresponding angles are

equal. In other words, in a set of similar triangles, one triangle is simply an enlarged version of the other.

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2

Similar triangles are often used in proving the Pythagorean Theorem, as they will be in this proof. In this

proof, we will ﬁrst compare similar triangles ABC and CBD, then triangles ABC and ACD.

Comparing Triangles ABC and CBD

In the diagram above, side AB corresponds to side CB. Similarly, side BC corresponds to side BD, and

side CA corresponds to side DC. It is possible to tell which side corresponds to the appropriate side on

a similar triangle by using angles; for example, corresponding sides AB and CB are both opposite a right

angle.

Because corresponding sides are proportional and have the same ratio, we can set the ratios of their lengths

equal to one another. For example, the ratio of side AB to side BC in triangle ABC is equal to the ratio of

side CB to corresponding side BD in triangle CBD:

length of CB

length of AB

=

length of BC

length of BD

Written with variables, this becomes:

c

a

=

a

x

Next, we can simplify this equation by multiplying both sides of the equation by <math>a</math> and

<math>x</math>:

x×a×

a

c

= ×x×a

a

x

With simpliﬁcation, we obtain:

cx = a2

Comparing Triangles ABC and ACD

Triangle ABC is also similar to triangle ACD. Side AB corresponds to side CA, side BC corresponds to side

CD, and side AC corresponds to side DA.

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Using this set of similar triangles, we can say that:

length of CA

length of AB

=

length of DA

length of AC

Written with variables, this becomes:

b

c

=

c−x

b

Similar to before, we can multiply both sides of the equation by <math>c-x</math> and <math>b</math>:

b × (c − x) ×

b

c

= × b × (c − x)

c−x

b

b2 = c(c − x)

c2 = cx + b2

Earlier, we found that cx = a2 . If we replace cx with a2 , we obtain c2 = a2 + b2 . This is just another way

to express the Pythagorean Theorem. In the triangle ABC, side c is the hypotenuse, while sides a and b

are the two legs of the triangle.

A Debate about True Origins

Although the theorem has been attributed to and named after Pythagoras and his community of scholars,

it is believed that the concepts behind the theorem were known long before the Pythagoreans proved it.

Among historians, an ongoing debate ensues about the possibilities that the ideas behind the Pythagorean

Theorem were independently discovered by diﬀerent groups at diﬀerent times. A wide variety of theories

exist, but there is substantial evidence that various civilizations used the Pythagorean Theorem, or were

at least aware of the main principles of the theorem, to ﬁnd the side lengths of right triangles.

Babylonians

In 1800 BCE, more than a thousand years before Pythagoras founded his school, a group of people living in

Mesopotamia (located in present-day Iraq) already understood the relationship between the side lengths of

a right triangle. These people, called Babylonians, were the ﬁrst known group to demonstrate a conceptual

understanding of the Pythagorean Theorem.

Historians have gained an understanding of the Babylonians from studying the ancient clay tablets they

have left behind. These tablets were used throughout Mesopotamia to record a variety of information about

commerce, culture, and daily life. Two of these clay tablets have particular relevance to the Pythagorean

Theorem. On one of these tablets, which has been named YCB (short for Yale Babylonian Collection)

7289 since its discovery, there is an illustration of a tilted square with its two diagonals drawn in. In their

own numeration system, Babylonians labeled the sides of the square as having a length equivalent to the

value of 1 in our number system and the√diagonal with a length equivalent to 1.414213. This decimal is a

miraculously accurate approximation of 2 , which proves that the Babylonians had very reﬁned methods

of calculation.

The Pythagorean Theorem was never explicitly written on any of the recovered clay tablets, but the

engravings on tablet YBC 7289 display an early understanding of the Pythagorean Theorem because the

diagonal of the square can be thought of as the hypotenuse of a right triangle. The legs of this right triangle,

which are simply the sides of the square, each have a length of 1. By the Pythagorean Theorem, of which

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4

Figure 1.1: Plimpton 322 tablet with engravings of Pythagorean triples.

√

the Babylonians must have had some understanding,

the

diagonal

must

have

a

length

of

12 + 12 (because

√

2

2

2

(leg1 ) + (leg2 ) = (hypotenuse) ). This is simply 2 or, as the Babylonians approximated, 1.414213.

A second tablet (shown in Figure 1.1), named Plimpton 322 after the collection to which it belongs,

reveals the Babylonians’ advanced understanding of right triangles. Inscribed in this tablet is a table of

Pythagorean triples, which are sets of three positive integers (a, b, c) that would satisfy the Pythagorean

Theorem (a2 + b2 = c2 ). One example of a Pythagorean triple is the set (3, 4, 5), as seen in Example 1.

We will explore Pythagorean triples more fully in the chapter “Applying the Pythagorean Theorem.”

Egyptians

Like Mesopotamia, Egypt was a great ancient civilization whose inhabitants were very commercially and

culturally advanced. The Egyptians never explicitly expressed the Pythagorean Theorem as we know it

today, but they must have used it in constructing their pyramids. It is known that, when building the

pyramids, Egyptians used a knotted rope as an aid in making right angles. This rope had twelve evenly

spaced knots (similar to the diagram below) that could be formed into a 3-4-5 right triangle with one angle

of 90◦ . The ropes were used as a model for the much larger right triangles used in the pyramids, which

were built during a period of 1,500 years as a way to honor the pharaohs.

5

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