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6 1. Revision Description Recall basics of geometry Reflect and Review Teasers Answers The Front, Top and Side view 1) Write the name of the of the solid shape polygon shown below. 1) Hexagon 2) 41 cm are as follows: 2) Find the diameter of a circle of radius 20.5 cm. Front view Top view Side view 2. Point, Line, Line Segment and Ray Description Reflect and Review A point is a basic unit of geometry. A point represents position only; it has zero size. We represent a point by a dot and name it with a capital letter, say A. A Teasers List some examples from your daily life which reminds you of a line, a line segment and a ray. 1 A line is a collection of points extending endlessly in both the directions along a straight path. A line has no starting point and no ending point. A B Here, ⃡ is a straight line. The arrows in two directions indicate that it goes endlessly in both the directions. A line can also be represented by k, l, m, n, etc. A part of a line is called a line segment. It has a starting point and an ending point. A B We call a line segment by means of its starting point and its ending point as line segment AB, and write it as ̅̅̅̅ or ̅̅̅̅. A ray is a part of line which has a starting point but goes endlessly in the other direction. A B The direction in which the ray goes endlessly and the starting point are clearly indicated in the ray, and is written as . 2 3. Angle Description Reflect and Review Teasers Answers Angle When two rays meet The angle formed at O is written as ∠AOB or ∠BOA, where ∠ is the symbol at a point, an angle for an angle. is formed. The point where the rays meet is called the vertex and the rays are called the arms of the angle. A Angle A degree is the unit of measurement used for measuring angles and it is symbolically denoted as °. O B Measuring Angles We can use a protractor to measure angles. A protractor has two sets of measurements written on it. These are called scales. There is an inner scale and an outer scale, both having 0° to 180° in different directions. There is a baseline and the midpoint of the baseline is called centre. Types of Angles Measuring an angle ∠ C: Step 1: Place the protractor over ∠ABC such that the centre point coincides with the vertex B of the angle and the baseline lies on the arm BC of ∠ABC. Draw an angle using scale and pencil, and find its measure using a protractor. Step 2: Note down the measure of the angle against the arm BC counting from 0°. Thus, ∠ABC = 600. 1) Measure 1) 900 3 4 An angle measuring less than 900 is called an Acute angle. An angle measuring exactly 900 is called a Right angle. An angle measuring greater than 900 is called an Obtuse angle. An angle measuring exactly 1800 is called a Straight angle. An angle measuring exactly 3600 is called a Complete angle. the angle formed between the two corners edges of you note book. 2) Give some examples of acute angles formed while writing English alphabets . 2) A, K, M, N, V, X, Y, Z. 4. Perpendicular and Parallel Lines Description Reflect and Review If the angle between two intersecting lines is 900, then the lines are said to be Perpendicular to each other. List some parallel and perpendicular lines that you see in your day to day life. P The symbol ⊥ is used to denote perpendicular lines. Q O Teasers R In the figure, OP is perpendicular to QR. It is written as: OP ⊥ QR. If two lines never meet each other and are at same distance always, then they are said to be Parallel lines. The symbol || is used to denote parallel lines. C A D B In the figure, line AB is parallel to line CD. It is written as: AB || CD. 5