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Try to find out what this is :

  1. Apr 3, 2005 #1
    Take a sheet of paper, a straightliner and a compass.

    Make a cylinder out of the sheet of paper and use the compass to draw on the cylinder a "circle" with the same radius as the cylinder (let's put that radius R=1(whichever unit you want)).

    Unfold the sheet and take the big axis of the closed curved obtained. One can show, this length is : [tex] L=\frac{\pi}{3} [/tex].

    Triple this distance with compass, and you can now construct a square which perimeter is [tex]4\pi[/tex]

    Now construct a circle of radius 2, and you built a circle of same perimeter as the square above....
  2. jcsd
  3. Apr 3, 2005 #2


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    wait how do you draw a circle on a cylinder? do you mean to go all the way around? because that would give you only [tex] 2 \pi r [/tex] i dont see where the [tex]\pi/3[/tex] comes from...

    *edit* maybe you mean to draw it on sideways, and even then, why would you want a square with the same perimeter as a circle? forgive the naiveness of my questions :biggrin: (also, exactly how do you draw a circle on sideways like that if you dont want to crush your paper cylinder?)
  4. Apr 3, 2005 #3
    Well the point is exactly that taking the compass (carefully) on the cylinder doesn't draw a circle (neither an ellipse by the way).

    If you want : look at the cylinder from the basis circle : take the radius of the cylinder on your compass : put (carefully), the compass on the surface of the cylinder. You immediatly see that this does not make the complete turn around of the cylinder, but only 60°=Pi/3.....Is that clearer ?

    Since you now unfold the paper, u get a length that is multiple of Pi u did the quadrature of the circle.
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