Page 228 - ICSE Math 7

Point to remember

If ∠A, ∠B and side AC of DABC are given, then first calculate ∠C and then construct the required
triangle. Thus, if a side and any two angles of a triangle are given, then first calculate the base angle
and then construct the triangle.

To construct a triangle when its three sides are given
Let’s construct a DABC in which AB = 5.5 cm, BC = 8.2 cm and CA = 7.4 cm. A 5.5 cm B
To do so, follow the steps given below. C
Step 1: Draw a line segment AB of length 5.5 cm.
Step 2: With A as centre, draw an arc of radius 7.4 cm. 7.4 cm 8.2 cm

Step 3: With B as centre, draw an arc of radius 8.2 cm to intersect the arc
drawn in step 2 at C.
Step 4: Join A and C, and B and C to get the required DABC. A 5.5 cm B

To construct an equilateral triangle when its side is given
Let’s construct an equilateral DABC with side 5 cm. To do so, follow the steps given below.

Step 1: Draw a line segment AB of length 5 cm. A B
5 cm
C

Step 2: With A as centre, draw an arc of radius 5 cm.
Step 3: With B as centre, draw an arc of radius 5 cm to cut the arc drawn 5 cm 5 cm
in step 2 at C.
Step 4: Join A and C and B and C to get the required equilateral DABC.
A B
5 cm
To construct an isosceles triangle when its base and base angles are given
Let’s construct an isosceles DABC with AB = 7.4 cm, ∠CAB = ∠CBA = 60°.
To do so, follow the steps given below.
Step 1: Draw a line segment AB of length 7.4 cm. A 7.4 cm B

D

Step 2: Draw ∠DAB = 60° at A.

A 7.4 cm B

E D
Step 3: Draw ∠EBA = 60° at B to intersect ray DA at C. C
DABC is the required isosceles triangle.

A 7.4 cm B

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