Circle vs. Square

Geometry Level 1

If the circumference of a circle and the perimeter of a square are equal, then the area of the circle is _ _ _ _ _ _ __ the area of the square.

Greater than Equal to Less Than

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2 solutions

If r r is the radius of the circle and x x the side length of the square, then we are given that

2 π r = 4 x x = π 2 r x 2 = ( π 2 ) 2 r 2 . 2\pi r = 4x \Longrightarrow x = \dfrac{\pi}{2}r \Longrightarrow x^{2} = \left(\dfrac{\pi}{2}\right)^{2}r^{2}.

But since π < 4 \pi \lt 4 we know that ( π 2 ) 2 = π 2 4 < π 2 π = π \left(\dfrac{\pi}{2}\right)^{2} = \dfrac{\pi^{2}}{4} \lt \dfrac{\pi^{2}}{\pi} = \pi , and so

x 2 = ( π 2 ) 2 r 2 < π r 2 x^{2} = \left(\dfrac{\pi}{2}\right)^{2}r^{2} \lt \pi r^{2} ,

i.e., with the given condition, the area of the circle will be greater than that of the square.

In the cobwebs of my mind, I remember in calculus 101, that the reason food containers (and other storage containers) are cylinders is: the maximum volume with minimum surface area is a cylindrical shape. Thereby maximizing volume while minimizing cost of material to produce the container.

A sphere would be best, but they don't pack or stack as well as cylinders.

Michael Rosen - 3 years, 11 months ago
Ankit Alria
Jan 30, 2016

a(side of square)=11r(radius of cirle)/7 therefore area of circle=3.1r(square) and area of square=2.4r(square)

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