Page 144 - New Medley 5 Semester 2

To measure an angle, we use an instrument called a protractor. You might
have seen this instrument in your geometry box. It is semicircular in shape.

Sometimes, it is also called D.

When you observe a protractor carefully, you find

that there are two sets of measurements written

on it. These are called scales. There is an inner
scale and an outer scale, each of them has marking

from 0° to 180° in opposite directions. There is a
baseline. It is the diameter of the semicircular instrument. The mid-point

of the diameter or baseline is called centre point of the protractor.

When we measure an angle, we place the centre point over the vertex of

the given angle and cover one arm with either side of the baseline. Then,
we read the marking obtained on a scale aligning with other arm. This is
the measure of the angle.

C

A
B

Here, the measure of ∠BAC is 50°.

Types of Angles

Observe the gap or opening between two matchsticks.

90° < < 180°

< 90° 90°
0°

On the basis of sizes, angles are classified as:
180°
  Zero angle: The angle whose measure is 0° is
called zero angle.

  Acute angle: The angle whose measure is greater than 0° and less than
90° is called acute angle.

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