~ Subtopic : Angles~ Learning Outcomes :

Students should be able to :1. Recognise angles2. Denote and label angles3. Measure angles using protractors4. Draw angles using protractors5. Recognise, compare and classify angles as acute, right, obtuse and reflex.

CAN YOU RECOGNISE STRAIGHT LINES IN THIS PICTURE?

LINES AND ANGLES

When two straight lines intersect,

they form an angle.

LOOK AROUND AND OBSERVE THE DESIGNS AROUND US. COULD YOU IDENTIFY MORE ANGLES?

1. Recognise angles Join two straight lines A (red line) and C (black line) at one

end (point B). Turn line A in an anticlockwise direction. Result:

~An angle is formed when the two lines meet at point B. ~The point B is called a vertex.

ABC

The angle can be written or labelled as B or ABC or CBA

2. Denote and Label Angles

Oral Exercise:Textbook – page 93 , Exercise 1 , no. 1.(a),(b),(c)

A

CBAngleVerte

x

The instrument to measure an angle is aHow To Measure An Angle?

3. Measure The Angle.

Outer scaleCentreInner scale

70

170

160

150

140130

120110

100908070

6050

40

30

20

10

1800

160

170 10

20

3040

5060

8090100110120

130140

150

180 0

70°

P

R

Q

Measuring an angle using the Inner scale Inner scale is marked ANTI-CLOCKWISE from 0o to 180o

Example:

So the answer for <PQR is 80o

The center must be on the vertex

P

QR

80o

Measuring an angle using the Outer Scale Outer scale is marked CLOCKWISE from 0o to 180o

Example:

40o

B

A

C

The center must be on the vertex

ABCSo is 40o

4. Drawing an angle using a

protractor.

C

Draw an angle of 50°. Steps: 1. Draw a straight line AB

2. Place a protractor with its center exactly on point B. 3. Now read the inner scale from 0o to 50o.

4. Make a small mark at 50o and name it C.5. Join point C to point B with a straight line.

B A 50ABC

Draw an angle of 150o

150o

Class activity

a)30o

b)45o

c) 75o

d)100o

e)120o

Construct the angles given below:

An angle less than 90o is an acute angle

An angle more than 90o but less than 180o is an obtuse angle

An angle more than 180o but less than 360o is a reflex angle

75o

120o

230o

5. Recognise, compare and classify angles as acute, right, obtuse and reflex.

Attention : Please refer text book on page 94.Exercise : Classify the angles in the diagram.

An angle of 90o is a right angle

Do this exercise orally :

Exercise 2, page 95.

*symbol

90o

1. Draw an acute angle of 300

6. Draw acute, obtuse, reflex and right angles.

300

2. Draw an obtuse angle of 1400

1400

2. Draw a reflex angle of 2200

1800 + 400 = 2200

2200

4. Draw a right angle.

Question : Calculate the sum of the two angles

7. Determine the angle on a straight line.

The sum of angles on a straight line is 1800

S

RQP PQS = 400 , SQR = 1400

Therefore, PQS(400) + SQR(1400) = 1800

90o

180o

270o 360o

8. Determine the angle of one whole turn. 0o

One whole turn is 360o

x310o

What is the value of x?

x = 50o= −

360o

x

310o

Try this out !

xx

xA B

AB is a straight line. Find the value of x.

x + x + x = 180o

3x = 180o

x = 180o÷ 3

x = 60o

PQ and RS are straight lines. What is the value of m?

m

135oP

RQ

S

m + 90o = 135o

m = 135o – 90o

m = 45o

Exercise (Refer Text Book):1)Page 95 Exercise 2 : No.1 a).b),c),d) ;

No. 2 a),b),c),d),g),h).2)Page 97 Exercise 3 : No.1,2,3,4 a),b),c).3)Page 98 Exercise 4 : No.1 a),b),c).4)Page 99 Exercise 5 : No.1a),b),d),e),f) ; No.2a),b),c),d).5)Page 105 Exercise 9 : No.1a),b),c),d) ; No.2a),b),c),d) ; No.3a),b),c),d).