# Understanding elementary shapes

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Corners, edges, planes, open curves and closed curves are different shapes that we see around us. Understanding these shapes can help us solve a wise variety of real world problems.

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An angle smaller than a right angle is called an acute angle, an angle greater than right angle but less then straight angle is obtuse angle while a reflex angle is larger then a straight angle. I this chapter we will also learn how to categorize triangles based on the angles that it has.

One complete revolution is divided into 360 equal parts. Each part is a degree, and angles are measured in degrees. A right angle is 90 degrees and a straight angle is 180 degrees. In this topic we will learn how to measure angles in degrees. We will also learn how to use a protractor and to solve for unknown angles.

Triangles can be classified in different ways. In this tutorial, we'll learn about scalene, isosceles, equilateral, acute, right, and obtuse triangles.

Not all things with four sides and 90 degree angles are squares or rectangles! Sometimes the angles in these 4-sided shapes are less than (or greater than) 90 degrees, causing them to have a slightly odd shape. These are called quadrilaterals and come in a few different varieties. In this tutorial, you'll learn about them.