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Knowledge Desk

Geometry was born from a practical need. In ancient Egypt,
the Nile River would flood each year, washing away property
boundaries. The Egyptians appointed special surveyors called FACTS
"rope-stretchers" who used knotted ropes to measure out The Great Pyramid of Giza
right-angled triangles and accurately redraw the perimeters has a perimeter of nearly
of farmlands. This was one of the first known uses of practical one kilometre (921.6 m).
geometry.

perimeter of an equilateral triangle

We know that a triangle in which all three sides are of equal Math Insight
length is called an equilateral triangle. Always assign a
In the adjoining figure, ABC is an equilateral triangle whose sides respective unit to
the final answer.
are AB = BC = CA.
Perimeter of a triangle = Sum of the lengths of its sides A
= AB + BC + CA

= AB + AB + AB (As AB = BC = CA)
= 3 × AB

Thus, the perimeter of an equilateral triangle = 3 × side B AB = BC = CA C
Example 10: Find the perimeter of an equilateral triangle whose side is 7 cm.

Solution: Perimeter of an equilateral triangle = 3 × side
= 3 × 7 c m
= 2 1 c m

Thus, the perimeter of the given equilateral triangle is 21 cm.
Example 11: The perimeter of an equilateral triangular-shaped signboard is 96 cm. Find
the length of its side.

Solution: Given, the perimeter of an equilateral triangular-shaped signboard = 96 cm.

Perimeter of an equilateral triangle = 3 × side
3 × side = 96 cm

s i d e = 96 = 32 cm
3
Thus, the length of the side of the signboard is 32 cm.

Challenge Question Critical Thinking

Find the length and breadth of a rectangular garden whose perimeter is 36 metres and
whose area is 80 square metres.

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