Subject : Mathematics

Topic : Geometrical Construction

Form : Two

Duration : 45 minutes

Learning Objective: Constructions Using a Straight Edge and a Pair of Compasses

Learning Outcomes :

At the end of lesson, students should be able to

• Construct the angle of 60 and 120 • Construct bisector of an angle.

Prerequisite knowledge :Students have already know • Construct a line segment of given length.• Construct a triangle given the length of the sides.• Construct a perpendicular to a line.

The perpendicular bisector of a given line segment. The perpendicular to a line through a point on the line. The perpendicular to a line passing through a point not on the line.

Approach : Constructive Learning

A

4. Then, label point C at the 60 mark using the outer scale. Remove the protractor. Join BC.

Draw the angle of 60 using the protractor

B

C

60

1. Draw a line AB.2. Place the base line of the protractor on AB.3. Place the centre of the protractor on the point B.5. An angle of 60 is drawn.ABC = 60

Construct an angle of 60°

Construct ABC = 60°

B C

A

60°

(a) Draw a straight line and mark the point B.(b) With B as the centre, use a compass to draw a big arc to cut the line at C.

(c) With the same radius and with C as the centre, cut the first arc at A.

(d) Join A to B. ABC = 60°

LMN = 60° PQR = 60° P

60°

L

60°

Draw a line AB of 5 cm. Construct an angle of 60° at A.

A B

60°

5 cm

Construct an angle of 120°

Construct BAD = 120°

A B

C

120°

D

BAD = 120° (a)Draw a straight line and mark the point A.(b) With A as the centre, use a compass to draw a big arc to cut the line at B.(c) With the same radius and with B as the centre, cut the first arc at C.(d) With the same radius and with C as the centre,

cut the second arc at D.(e) Join A to D.

Construct an angle of 120°

Construct ABD = 120°

B C

A

60°

D

120°

(a)Draw a straight line and mark the point B.(b) With B as the centre, use a compass to draw a big arc to cut the line at C.

(c) With the same radius and with C as the centre, cut the first arc at A.

(d)Join A to B.(e)Mark the point D at the other side of the line.ABD = 120°

Draw a line PQ of 6 cm. Construct an angle of 120° at Q.

P Q

60°

6 cm

XYZ = 120° MLN = 120°

Z

120°Z

120°

120°

Construct the bisector of an angle

Construct the bisector of ABC.

B

A

C

P

Q

R

(a) With B as the centre, use a compass to draw arcs to cut AB and BC at P and Q.(b) Without changing the radius and with P and then Q as the

centre, draw arcs to cut at R.

(c) Join R to B. BR is the bisector of ABC.

Construct an angle of 30°

Construct an angle of 60°

30°

then construct the angle bisector.

Construct an angle of 45°

Construct an angle of 90°

45°

then construct the angle bisector.

15°

15°

22 °

22.5°

2

1

150°

30°150°

75°

C

120°

D