 # Understanding Elementary Shapes

## The part of a line with two fixed points is called a line segment. A line segment has a length but no thickness. Methods for comparing the lengths of the line segments are: Comparison by observation Comparison by observation Comparison by observation An angle is the figure formed by two rays sharing a common endpoint, called the vertex of the angle. Angles can be measured in degrees using a protractor. Right angle: Angle of measure 900 Straight angle: Angle of measure 180° Complete angle: Angle of measure 360° Acute angle: Angle which is less than 90° Obtuse angle: An angle which is greater than 90° Reflex angle: A reflex angle is greater than a straight angle. Two intersecting lines are perpendicular to each other, if they form an angle of 90°. Perpendicular passing through mid-point of a line which divides a line in two equal parts is called perpendicular bisector of the line. A triangle is a closed shape with three sides. Classification of a triangle on the basis of its sides: Scalene Triangle: A triangle which has all unequal sides. Isosceles Triangle: A triangle which has two equal sides. Equilateral Triangle: A triangle which has all sides equal. Classification of a triangle on the basis of its angles: Acute Angled Triangle: It has all angles less than 90°. Right Angled Triangle: It has one angle equal to 90°. Obtuse Angled Triangle: It has one angle greater than 90°. A quadrilateral is a polygon which has 4 sides. Quadrilaterals are classified as follows: Square: A square has all four right angles and all four sides equal to each other. Rectangle: A rectangle has all four right angles and opposite sides equal to each other. Parallelogram: A parallelogram has opposite sides parallel to each other. Rhombus: A rhombus has all four equal sides and opposite sides are parallel to each other. Trapezium: A trapezium has one pair of parallel sides. A polygon is traditionally a plane figure that is bounded by a closed path or circuit, composed of a finite sequence of straight line segments. Polygons can be classified according to the number of their sides: Triangle: A triangle is a polygon which has 3 sides. Quadrilateral: A quadrilateral is a polygon which has 4 sides. Pentagon: A pentagon is a polygon which has 5 sides. Hexagon: A hexagon is a polygon which has 6 sides. Octagon: An octagon is a polygon which has 8 sides. Any object that has height, width and depth is a three-dimensional shape. Cube, cuboid, sphere, cylinder, cone, prism and pyramid are some of the three dimensional shapes. Each side of a 3-dimensional shape is a flat surface that is called a face. A line segment, at which two faces meet, is called an edge. When three edges meet at a point, it is called a vertex.

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