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Construct an equilateral triangle, given its side and justify the construction.

 

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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 60º. The steps mentioned below will be followed to draw an equilateral...
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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 60º.

The steps mentioned below 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. ΔABC 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 = 60°.

In ΔABC, we have AC = AB = 5 cm and ∠A = 60°.

Since AC = AB,

∠B = ∠C (Angles opposite to equal sides of a triangle)

In ΔABC,

∠A + ∠B + ∠C = 180° (Angle sum property of a triangle)

⇒ 60° + ∠C + ∠C = 180°

⇒ 60° + 2 ∠C = 180°

⇒ 2 ∠C = 180° − 60° = 120°

⇒ ∠C = 60°

∴ ∠B = ∠C = 60°

We have, ∠A = ∠B = ∠C = 60° ... (1)

⇒ ∠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), ΔABC is an equilateral triangle.

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Maths tutor with 4 years experience

The steps of construction are as follows: 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: Now taking P as centre draw an arc to intersect the previous arc at E. Join AE. Step III: Taking A as centre draw an...
read more
The steps of construction are as follows:

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: Now 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:
To justify the construction, we have to prove that ABC is an equilateral triangle i.e.AB = BC = AC = 5 cm and A = B = C = 60o.
 
Now, in ABC, we have AC = AB = 5 cm and A = 60o
Since, AC = AB, we have
B = C                         (angles opposite to equal sides of a triangle)
Now, in ABC
A + B + C = 180o    (angle sum property of a triangle)
60o + C + C = 180o
60o + 2 C = 180o
 2 C = 180o - 60o = 120o
 C = 60o
 
Now, we have A = B = C = 60o              ... (1)
 
A = B and A = C
BC = AC and BC = AB          (sides opposite to equal angles of a triangle)
AB = BC = AC = 5 cm                                  ... (2)
Equations (1) and (2) show that the ABC is an equilateral triangle.
 
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Draw a line segment AB of length equal to the length of the side of the triangle. At A draw angle of 60 degree. With A as centre and opening of compass equal to the given side cut an arc at C on the arm of the angle. Join BC. ABC Is the required triangle. All the three angles being equal to 60 degree...
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