Pythagoras Theorem

< 23.6 Triangle Concepts Summary(A) | Topic Index | 23.8 Pythagoras Theorem – A Visual Proof >

Pythagoras Theorem is easily one of the most famous equations in science &amp; mathematics which are familiar to anyone who has attended school. It states that in any right-angled triangle, the sum of the squares on the 2 smaller sides would be equal to the sum of the square of the hypotenuse.

Discovered in Many Cultures

This theorem was known even to Sumerians, Babylonians, Chinese and the Hindus.

Egyptians used it for measuring out their fields in rectangular shapes. They used a looped rope 12 units long knotted so that when stretched, it formed a triangle with sides 3, 4 & 5. These persons were even called "rope stretchers"!

Many cultures knew of different sets of 3 numbers (3, 4 & 5 or 5, 12 & 13), or Pythagorean Triples which could form right-angle triangles. But they possibly did not know why such numbers formed a right-angle triangle!

Proof of the Pythagoras Theorem

The credit for finding this goes to the Greeks. They "proved" the theorem. They took results about lines, angles etc which had been proved earlier and from these results, proved the theorem. This result then became common intellectual property.

We have no conclusive evidence as to who proved the theorem first. In fact there is no evidence as to how the theorem got associated with Pythagoras.

Euclid includes the proof of the Pythagoras Theorem and its converse as proposition 47 in his Book 1. But he did not call it the Pythagorean theorem. .

The logical simplicity and beauty of his proof has excited many to invent their own proofs. Some of the were geometric and some were algebraic. In 1968, the National Council of Math Teachers (NCTM) published a book listing 367 proofs of the theorem. We give one such proof at the end of this article.

Importance of the Right Angled Triangle

The special relationship between the sides of a right angle convinced Greek mathematicians of the importance of the right angled triangle.

There is one more reason which convinced them of this.

Most shapes in this world could be approximated with polygons and any polygon can be subdivided into a number of triangles. Each triangle can be divided into two right angled triangles by drawing one of the altitudes. Any right angled triangle can be divided by drawing an altitude meeting the hypotenuse into two similar right angles.

Hence any shape can be divided into a series of right angled triangles! Hence the world is made of right angled triangles!

Numbers Vs Segments

Pythagoras theorem does not really talk in terms of lengths of the sides of the triangle, but in terms of areas of squares on the sides.

This is because in ancient times, number systems had not progressed beyond rational numbers. Lengths of line segments were denoted by the term "magnitude" and were considered a different kind of quantity other than numbers. Therefore, they worked only with ratios of segments.

Proving the Pythagoras Theorem using Areas

Also the Pythagoras theorem was easy to prove using the areas of squares on its sides.

If we drawn a right angled triangle with sides 3, 4 & 5on a square ruled paper, using simple geometry, we can see that the squares on each of these sides would have areas 9, 16 & 25 and 9 + 16 = 25.

Lengths of Line Segments

In coordinate geometry, the Pythagoras Theorem gave an easy way to find the distance of an line segment connecting any two points on the cartesian plane.

Extension of Pythagoras Theorem

Once the theorem can be thought of as a relation between areas, it can also be seen as a relation between areas of similar figures, not only squares. Hence the area of a circle drawn with the hypotenuse of a right-angle triangle, will be equal to the sum of the areas of circles drawn on the other two sides! If 3 similar triangles are drawn on the 3 sides of a right-angle triangle, then the area of the triangle on the hypotenuse will be equal to the sum of the triangles on the other 2 sides.

Pythagorean triples

Any 3 numbers which satisfy the relation of the theorem are called &ldquo;Pythagorean Triples&rdquo;. (3, 4 &amp; 5), (5, 12 &amp; 13) and (8, 15 &amp; 17) are some examples. In fact, using the following 3 algebraic expressions given below will give an infinite number of Pythagorean triples for different values of m &amp; n.

(m2 - n2) ,mn, (m2 + n2)

Proof of Pythagoras Theorem

This proof is simple. It can be demonstrated with a sheet of paper which can be cut and rearranged to bring out the proof.

Chapter 23.8 gives a visual proof of the theorem.

< 23.6 Triangle Concepts Summary(A) | Topic Index | 23.8 Pythagoras Theorem – A Visual Proof >