NCERT Solutions
Class 9 Maths
Constructions

Ex.11.1 Q.5
Construct an equilateral triangle, given its side and justify the construction.
Let us draw an equilateral triangle of side 5 cm. We know that all sides of an equilateral triangle are equal.
Therefore, all sides of the equilateral triangle will be 5 cm. We also know that each angle of an equilateral triangle is 600.
The below given steps will be followed to draw an equilateral triangle of 5 cm side.
Step I: Draw a line segment AB of 5 cm length. Draw an arc of some radius, while taking A as its
centre. Let it intersect AB at P.
Step II: Taking P as centre, draw an arc to intersect the previous arc at E. Join AE.
Step III: Taking A as centre, draw an arc of 5 cm radius, which intersects
extended line segment AE at C. Join AC and BC.
∆ABC is the required equilateral triangle of side 5 cm.
Justification of Construction:
We can justify the construction by showing ABC as an equilateral triangle i.e.,
AB = BC = AC = 5 cm and ∠A = ∠B = ∠C = 600.
In ∆ABC, we have AC = AB = 5 cm and ∠A = 600.
Since AC = AB, ∠B = ∠C
[Angles opposite to equal sides of a triangle]
In ∆ABC,
∠A + ∠B + ∠C = 180°
[Angle sum property of a triangle]
∠ 600 + ∠C + ∠C = 1800
∠ 600 + 2 ∠C = 1800
2 ∠C = 1800 − 600 = 1200
∠C = 1200/2
∠C = 600
∠B = ∠C = 600
We have, ∠A = ∠B = ∠C = 600 …………......(1)
So, ∠A = ∠B and ∠A = ∠C
BC = AC and BC = AB
[Sides opposite to equal angles of a triangle]
∠ AB = BC = AC = 5 cm ..........................(2)
From equations 1 and 2, ∆ABC is an equilateral triangle.