Page 45 - Math_Genius_V1.0_C6_Flipbook

E:\Working\Focus_Learning\Math_Genius-6\Open_Files\02_Chapter_2\Chapter_2
\ 07-Nov-2024 Bharat Arora Proof-8 Reader’s Sign _______________________ Date __________

For example, to measure ∠ABC, place the protractor overlapping base BC aligned with the
horizontal line with point B at the centre of the protractor. Look at the second arm of ∠ABC i.e.,
AB adjusted to overlap with a line marking on the protractor. Here it coincides with 40 degree
making of inner scale, so ∠ABC = 40°.

Get it right!

l Angle opens towards right, so ∠PQR = 50°
using inner scale.
l Angle opens towards left, so ∠PQR = 130°
using outer scale.

How to measure an angle bigger than 180°?
Since the protractor (semicircular) is marked only between 0° to 180°, we can measure an angle
greater than 180° with a little extra effort. An angle that is more than 180° but less than 360° is
known as a reflex angle. You will learn more about it in next section.
Let us measure the reflex angle AOB.
A O

B

Step 1: Produce one of the arms to draw a straight line A O C
from the vertex of the angle. AO is extended to C.
B
Step 2: Measure the acute angle, ∠BOC and add 180° to it.

Here, ∠BOC = 20°

So, reflex angle ∠AOB = 180° + 20° = 200°

43 Lines and Angles