Class 6 Maths Chapter 5 Understanding Elementary Shapes Exercise 5.4 pdf notes:-

**Exercise 5.4** Class 6 maths Chapter 5 Pdf Notes:-

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## Ncert Solution for Class 6 Maths Chapter 5 Understanding Elementary Shapes Exercise 5.4 Tips:-

Measuring Angles

The improvised āRight-angle testerā we made is helpful to compare angles with

a right angle. We were able to classify the angles as acute, obtuse or reflex.

But this does not give a precise comparison. It cannot find which one among

the two obtuse angles is greater. So in order to be more precise in comparison,

we need to āmeasureā the angles. We can do it with a āprotractorā.

The measure of angle

We call our measure, ādegree measureā. One complete revolution is divided

into 360 equal parts. Each part is a degree. We write 360Ā° to say āthree hundred

sixty degreesā.

Think, discuss and write

How many degrees are there in half a revolution? In one right angle? In one

straight angle?

How many right angles make 180Ā°? 360Ā°?

Fold it twice to get a shape as shown. This is

called a quadrant.

- Open it out. You will find a

semi-circle with a fold in the

middle. Mark 90o

on the fold.

- Fold the semicircle to

reach the quadrant. Now fold the quadrant

once more as shown. The angle is half of

90o

i.e. 45o

Open it out now. Two folds appear on each side. What

is the angle upto the first new line? Write 45o

on the

first fold to the left of the base line.

- The fold on the other side would be 90o

- 45o

= 135o

- Fold the paper again upto 45Ā° (half of the

quadrant). Now make half of this. The first

fold to the left of the base line now is half of

45Ā° i.e. 22 1

2

o

. The angle on the left of 135o

would be 157 1

2

o

.

You have got a ready device to measure angles. This is an approximate

protractor.

**The Protractor**

You can find a readymade protractor

in your āinstrument boxā. The curved

edge is divided into 180 equal parts.

Each part is equal to a ādegreeā. The

markings start from 0Ā° on the right side

and ends with 180Ā° on the left side, and

vice-versa.

Suppose you want to measure an angle ABC.

- Place the protractor so that the mid point (M in the figure) of its

straight edge lies on the vertex B of the angle. - Adjust the protractor so that BC is along the straight-edge of the protractor.
- There are two āscalesā on the protractor : read that scale which has the

0Ā° mark coinciding with the straight-edge (i.e. with ray BC

). - The mark shown by BA on the curved edge gives the degree measure of

the angle.

We write m ā ABC= 40Ā°, or simply ā ABC= 40Ā°.