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\ 19-Nov-2025 Bharat Arora Proof-9 Reader’s Sign _______________________ Date __________

The measure of an angle is expressed in degrees (°). A complete turn about a point
is divided into 360 parts. Each part measures 1°. So, a complete turn about a point
measures 360°.

Protractor
Inner
A protractor is an instrument used to scale
measure an angle. It is semicircular in shape Outer
scale
80
with two edges — one semicircular and the 60 70 110 100 90 100 70 120
110
90
80
other straight. The semicircular edge has two 50 130 120 60 50 130
scales, an inner scale and an outer scale. Baseline 30 150 40 140 40 140 150
30
Angles from 0° to 180° are marked on 20 160 20 160
both the scales. We read the outer scale 10 170 10 170
clockwise and the inner scale anticlockwise. 0 180 0 180
The straight edge has a baseline. The Centre point
midpoint of the baseline is called the centre

point. B

To Measure an Angle using a Protractor

Let us measure ∠BAC with the help of a protractor.

Place the protractor on it in such a way that the centre point A C
lies on A, the vertex of ∠BAC, and the baseline coincides with
side AC of ∠BAC as shown in the figure. B
80
Now, note the marking along the edge of the 60 70 110 100 90 100 70 120
110
90
80
protractor through which the other side AB of 50 130 120 60 50
130
∠BAC passes. 30 40 140 40 140 150
30
Thus, ∠BAC = 70°. 20 160 150 20 160
10 170 10 170
180 0
0 A 180 C
Keep in Mind

• Start measuring the angle from the ray which is
R
90
80
aligned with the baseline of the protractor. 70 90 100 110
60 120 110 100 80 70 120
• Look at the scale on the protractor that 40 140 50 130 60 50 130 140
starts at the zero degree mark where the 30 150 40 30 150
baseline coincides with the ray and read 20 160 20 160
10 170 10 170
the measure of the angle where the other 0 180 0 180
arm crosses the scale. P Q
Here, ∠ PQR = 125°.

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Geometry