Page 43 - Mathematics Class - IX

PROCEDURE
1. Take a printout of full protractor and paste it on the cardboard as shown in Fig. (e).

2. Cut the two straws in halves along the length and fix them with nail or board pin at the centre O as shown
in Fig. (f).
P
90°
90º

O R 180° 0° S
180º 0º O

90°
90º
Q
Fig. (e) Fig. (f)

3. Name the two straw strips as PQ and RS.
4. Set the different positions of the strips to make vertically opposite angles and adjacent angles and write your
observations.

OBSERVATIONS

By actual measurement:
1. ∠POS = ____________ ∠SOQ = ____________ ∠QOR = ____________ ∠ROP = _____________

∠POS = ∠QOR =__________ and ∠SOQ = ∠ROP = _____________

2. ∠ROP + ∠POS = _____________, ∠POS + ∠SOQ = _____________
∠SOQ + ∠QOR = _____________, ∠QOR + ∠ROP = _____________

3. ∠POS + ∠SOQ + ∠QOR + ∠ROP = _____________

INFERENCE
Hence we verified that if two lines intersect, then

1. Vertically opposite angles are equal. 2. The sum of two adjacent angles is 180°.
3. The sum of all four angles is 360°.

EXTENDED TASK
1. Verify experimentally that if a transversal intersects two parallel lines, then

(i) Each pair of alternate interior angles is equal.
2. Prepare a presentation on all the properties of lines and angles with examples.

APPLICATION

Properties of lines are used in solving many geometrical problems.
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