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4. Name all the angles in the figures given below.
( a) (b)
A S
R
B T

O C P Q
5. Look at the given figure. Write: A

( a) the points that lie on the angle. K E
( b) the points that lie in the interior of the angle. H I

( c) the points that lie in the exterior of the angle. B D F G C

measurinG an anGle J

The measure of rotation between the arms of an angle gives the measure of the angle in
degrees (°). Observe the rotations of an arm of an angle.

B
B B

O A O A O A B O A

O O A O A O
A B A

B
B B

A complete turn of the arm about the vertex of the angle is divided into 360 equal parts.
Each part measures 1°. So, a complete turn about a point measures 360°.

The measurement of an angle is done using a protractor.
a protractor

A protractor is a D-shaped instrument used for measuring angles.
It is semicircular in shape and has two edges, one Inner

semicircular and the other a straight edge. The Outer scale
semicircular edge has two scales that are called the scale 70 80 90 100 110
60 120 110 100 90 80 70 120
inner and outer scales. Angles from 0° to 180° are Base 40 50 130 60 50 130 140
marked on both the scales. line 30 150 140 40 30 150
20 160 20 160
We read the outer scale clockwise and the inner scale 10 170 10 170
anticlockwise. The straight edge has a baseline. The 0 180 O 0 180 R
mid-point of the baseline is called its centre point. Centre

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