# Understanding Elementary Shapes

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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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