# 6 Math Chapter 5 Understanding Elementary Shapes

Chapter Notes and Summary
Length : distance between two end points of a line segment is called its length.
Protractor : It is an instrument used for measuring or constructing an angle.
Angle :
figure formed by two rays with same initial point is called an angle and it is measured in degree
(°)
Right angle : A quarter turn of one revolution by a ray is called a right angle i.e., 1 right angle = 90°.
(i) Acute angle :
An angle whose measure is more than 0° but less than 90° is called an acute angle.
(ii) Obtuse angle :
An angle whose measure is more than 90° but less than 180° is called an obtuse angle.
(iii) Straight angle :
An angle whose measure is 180°, is called a straight angle.
(iv) Reflex angle :
An angle whose measure is more than 180° but less than 360° is called a reflex angle.
(v) Complete angle :
An angle whose measure is 360°, is called a complete angle.
Polygon : A closed figure bounded by three or more than three line segments is called a polygon.
• If a polygon is bounded by 3 sides, it is called a triangle.
• If a polygon is bounded by 4 sides, it is called a quadrilateral.
Triangle : A plane figure bounded by three line segments is called a triangle and sum of angles of a triangle is 180°.
• If in a triangle each angle is acute, it is called an acute angled triangle.
• If in a triangle one angle is right angle, it is called a right angled triangle.
• If in a triangle, one angle is obtuse, it is called an obtuse angled triangle.
• A triangle having all three sides unequal is called a ‘scalene triangle.’
• A triangle having two sides equal is called an ‘isosceles triangle.’
• A triangle having all three sides equal is called an ‘equilateral triangle.’
Quadrilateral : A plane figure bounded by four line segments is called a quadrilateral.
• A quadrilateral having opposite pair of sides parallel is called a parallelogram.
• A quadrilateral having only one pair of parallel sides is called a trapezium.
• A quadrilateral having opposite sides equal and each angle of 90° is called rectangle.
• If a quadrilateral is having all sides equal and each angle of 90°, it is called a square.
• A parallelogram having all of its sides equal is called a rhombus.
Three dimensional shape :
Sphere : An object which is in shape of a ball. It has a curved surface but no vertex and no edge.
Cone : Objects in form of ice-cream cone, a conical tent, birthday cap etc.
Cylinder : A circular pipe, a circular pillar etc. are in shape of cylinder. It has one curved surface and two circular faces at its ends.
Cuboid : A brick, a matchbox, a book, etc. are in shape of a cuboid. It has length, breadth and height. It has 6 rectangular faces. Two faces meet of a line segment called an edge. Three edges meet at a point called vertex.
Cube : A cuboid whose length breadth and height are equal e.g., dice, ice cubes etc. A cube has 6 square faces, 12 edges and 8 vertices.
Pyramid : A solid whose base in a plane rectilinear figure and whose side faces are triangles having a common vertex.