Page 131 - ICSE Math 5

To measure an angle using a protractor, we follow the given steps.
• Keep the centre point of the protractor on the P
vertex of the angle.

• Adjust the protractor in such a way that the
baseline of the protractor is along one arm of
the angle, without disturbing the position of
the vertex.
R
• Look at the scale where the baseline arm points Q
to 0°, i.e., right or left of the vertex. In this case,
the baseline QR is towards right. So, we read Top Tip
the inner scale which starts from 0°.
• If the baseline is to the right of the
• Move along the inner scale starting from point vertex, we read the inner scale.
R till you reach point P. Read the marking at • If the baseline is to the left of the
point P as indicated by the other arm QP. It vertex, we read the outer scale.
reads 60°.
Thus, PQR = 60° (acute angle).

Now let’s learn to construct different angles using a protractor. For example, to construct an
angle of 70°, we follow the given steps.

Step 1: Draw a ray YZ of some length with the help
of a ruler. Y Z
Step 2: Keep the centre of the protractor at vertex
X
Y so that the baseline coincides with YZ .

Step 3: Now read the value on the inner scale of
the protractor where it points 70°. Mark it
with a dot as point X.
Y
Z

Step 4: Remove the protractor and join point X with
point Y. 6
Hence, XYZ = 70°

5 1cm2 X

4 5 4 3

3 6
70°
7
Y Z
8
9

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