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Sub. :- Mathematics Angle and Triangle Std. :- 5 th Chapter no. 12

See the following Pictures: The picture shows a corner formed by ladder and ground.

This picture shows a corner formed by seasaw and ground.

What is an Angle? AAn angle is the figure formed by two rays sharing a common endpoint. TThe common endpoint is called as Vertex. TThe word angle comes from the Latin word angulus, meaning "a corner".

Elements of an Angle AAbove figure shows an angle. RRays BA and BC having common endpoint B. TThe rays BA and BC are called as arms of an angle.

Name of an Angle NName of an Angle can be written by using three capital letters, as angle ABC or CBA. TThe letter naming the vertex of an angle is always written in the middle. It can be read as ‘angle ABC’ or ‘angle CBA’. NName of an angle also can be written by using a single letter, as angle B. TThe sign to denote an angle is ‘<’.

IIn above figure, there are three rays OA,OB and OC. ‘‘O’ is the common endpoint. TThere are three angles namely, ‘angle AOB’, ‘angle BOC’ and ‘angle AOC’.

EXERCISE

1.Which of the following figures shows angles? (a) (b) (c) (d)

2. Write the names of the angles in three letters and in one letter. (a) (b) (C) (d)

3. Write the names of all the angles in each of the figures below, using three letters. (a) (b) (c)

The protractor The protractor is an instrument used to measure a given angle in degrees or to draw an angle of given measure.

The protractor has the shape of semicircle. The semicircular edge is divided into 180 equal parts. Each part is called a degree. One degree is written as 1 0 The centre of a semicircle is called centre of the protractor. The diameter of a semicircle is called the reference line of the protractor. Reference line

To measure an angle using a protractor A B C Measure  ABC using protractor. Place the centre of the protractor on vertex B Hence ray BC coincide with the reference line. Now, the zero mark on the protractor will lie on ray BC. Which mark from the protractor coincides with ray BA ? It is the 65 0 mark. So the measure of  ABC is 65 0 i.e. m  ABC=65 0

m  AKV is 45 0 See how the angles in figure below are measured with the help of protractor. Example: 1

m  DEF is 90 0 See how the angles in figure below are measured with the help of protractor. Example: 2 D E F

m  STD is See how the angles in figure below are measured with the help of protractor. Example: 3 S TD

A B C D E F P Q R Measures the following angles with the help of protractor

1) m  DEF is 90 0 D E F

2) m  PQR is p Q R

3) m  ABC is 30 0 A B C

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