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Which one of the following has a greater perimeter than the rest?

  • A.A square with an area of 36 sq.cm
  • B.An Equilateral Triangle with a side of 9 cm
  • C.A rectangle with 10 cm as length and 40 sq.cm as area
  • D.A circle with a radius of 4 cm

Answer: C

Work out all four perimeters and compare.

Square, area 36: side \( \sqrt{36} = 6 \), so

\[ P = 4 \times 6 = 24 \text{ cm} \]

Equilateral triangle, side 9:

\[ P = 3 \times 9 = 27 \text{ cm} \]

Rectangle, length 10 and area 40: breadth \( \tfrac{40}{10} = 4 \), so

\[ P = 2(10 + 4) = 28 \text{ cm} \]

Circle, radius 4:

\[ C = 2\pi (4) = 25.13 \text{ cm} \]

The rectangle's 28 cm is the largest. It is also the least compact shape here, which is the general pattern: for a given area, the more elongated the figure, the longer its boundary - and the circle is always the most economical.

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