# Introduction

The word â€˜geometryâ€™ comes from the Greek words â€˜geoâ€™, meaning the â€˜earthâ€™, and â€˜metronâ€™, meaning â€˜to measureâ€™. Geometry appears to have originated from the need for measuring land. This branch of mathematics was studied in various forms in every ancient civilization, be it in Egypt, Babylonia, China, India, Greece, the Incas, etc. The people of these civilizations faced several practical problems which required the development of geometry in various ways.

The Greek mathematician, Thales is credited with giving the first known proof. This proof was of the statement that a circle is bisected (Example, cut into two equal parts) by its diameter. One of Thalesâ€™ most famous pupils was Pythagoras (572 BC), about whom you have heard.Â Pythagoras and his group discovered many geometric properties and developed the theory of geometry to a great extent. He demonstrated the following property of the right triangle which is called Pythagorean theorem.

Theorem: In a right triangle, the square of the length of the hypotenuse is equal to the squares of the lengths of the other two sides.

This process continued till 300 BC. At that time Euclid, a teacher of mathematics at Alexandria in Egypt, collected all the known works and arranged it in his famous treatise, called â€˜Elementsâ€™. He divided the â€˜Elementsâ€™ into thirteen chapters, each called a book. These books influenced the whole worldâ€™s understanding of geometry for generations to come.

In this chapter, we shall discuss Euclidâ€™s approach to geometry and shall try to link it with the present day geometry.

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**Euclid (325 BC - 265 BC)**