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The perimeters of a circle, a square and an equilateral triangle are equal. Which one of the following statements is true?

  • A.The circle has the largest area.
  • B.The square has the largest area.
  • C.The equilateral triangle has the largest area.
  • D.All the three shapes have the same area.

Answer: A

The circle has the largest area.

Give all three shapes the same perimeter P and compare their areas:

  • circle: radius \(P/2\pi\), area \(P^{2}/4\pi \approx 0.0796P^{2}\)
  • square: side \(P/4\), area \(P^{2}/16 = 0.0625P^{2}\)
  • equilateral triangle: side \(P/3\), area \(\frac{\sqrt{3}}{4}(P/3)^{2} \approx 0.0481P^{2}\)

The circle wins, and this is a general rule: among all plane figures with a given perimeter, the circle encloses the most area. The square comes second and the triangle last, so the statements naming the square or the triangle are false, and the areas are clearly not equal.

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