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Example 14: Which direction will we face if we start facing:

(a) West and make 1 of a revolution anti-clockwise?
2

1
(b) West and make 1 of a revolution clockwise?
2
1
Solution: (a) After of a revolution, we will face the East direction.
2
(b) In one revolution, we will face the West direction again. In
1
another revolution, we will face the East direction.
2
1
Hence, in 1 revolutions, we will face the East direction.
2

Example 15: In the figure given alongside:
(a) ∠AOD is a/an .................... angle.
D
(b) ∠COA is a/an .................... angle. E C

(c) ∠AOE is a/an .................... angle. 40°
40° 20° B
Solution: From the given figure, we find that
30°
(a) ∠AOD = 30° + 20° + 40° = 90°, so it is a right angle. O A

(b) ∠COA = 20° + 30° = 50°, so it is an acute angle.
(c) ∠AOE = 30° + 20° + 40° + 40° = 130°, so it is an obtuse angle.

Perpendicular Lines and Perpendicular Bisector N

We have learnt that two lines intersecting/meeting at a right angle are
known as perpendicular lines. The English alphabet ‘ T ’ is a common
example of perpendicular lines. W E

The line showing directions N-S is perpendicular to the line showing
directions E-W. So, we write NS ⊥ WE.
S
When a line is perpendicular to a line segment and bisects it into two
halves, it is called the perpendicular bisector of that line segment. Let M be the midpoint of
AB , and let line l intersect AB at a right angle passing through M. Then, line l is the perpendicular
bisector of AB .

l

A B A B
M M

55 Lines and Angles