Page 41 - Mathematics Class - IX

Unit 4 Geometryy
Geometr

ACTIVITY 4.1

OBJECTIVE
To verify experimentally that if two lines intersect, then
(a) Vertically opposite angles are equal (c) The sum of all the four angles is 360°
(b) The sum of two adjacent angles is 180°

MATERIALS REQUIRED
Cardboard Two straws A nail

White sheet of paper Printout of a full protractor Adhesive
PRE-REQUISITE KNOWLEDGE

1. Concept of vertically opposite angles 2. Concept of adjacent angles

THEORY
1. Line segment: A line with two endpoints. 2. Ray: The part of a line with one endpoint.
3. Angle: An angle is formed when two rays originate from the same endpoint.

4. Types of angles:
(i) Acute angle: An angle whose measure is more than 0° but less than 90°.
(ii) Right angle: An angle whose measure is 90°.
(iii) Obtuse angle: An angle whose measure is more than 90° but less than 180°.

(iv) Straight angle: An angle whose measure is 180°.
(v) Reflex angle: An angle whose measure is more than 180° but less than 360°.

y z
x

(i) acute angle: 0° < x < 90° (ii) right angle: y = 90° (iii) obtuse angle: 90° < z < 180°

s
t

(iv) straight angle: s = 180° (v) reflex angle: 180° < t < 360°
Fig. (a): Types of angles

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