Page 159 - Start Up Mathematics_5

To measure angles using a protractor, we follow these steps. P

Step 1: Keep the centre point of the protractor
on the vertex of the angle.

Step 2: Adjust the protractor in such a way that
the base line of the protractor is along
one arm of the angle, without disturbing
the position of the vertex. Q R
Step 3: Look at the direction of the baseline (right or Quick Tip
left of the vertex). In this case the base line QR is
towards right. So, we read the inner scale which • If the base line is to the right of
starts from 0°. the vertex, we read the inner
scale.
Step 4: Move along the inner scale starting from point R • If the base line is to the
till you reach point P. Read the marking at point left of the vertex, we read
P. It reads 60°. the outer scale.

Thus, ∠PQR = 60° (acute angle).

Measurement of some angles
X

• A right angle measures 90°.

Y Z

O P
• An acute angle is less than a right angle. So it is more than 0° but
less than 90°.

Q R

D

• An obtuse angle is more than a right angle. So it is more than 90° A
but less than 180°.
B C

W

• A straight angle measures 180°.

Construction of Angles Using a Protractor X Y Z
Let’s now learn to construct an angle using a protractor. To construct an angle of 60°, we
follow the given steps.

151