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Circle surface area

  1. Nov 21, 2014 #1

    How is circular surface area values square rooted ??. I am using this formula r2Xπ=mm2 to calculate the surface area of a circle but i want to know if there is a formula to get back to the diameter or the radius of the circle ?.

    example i have a circular surface area of 5.26mm2 and i want the diameter of that value, how do i solve this ?

    Thanks in advance
  2. jcsd
  3. Nov 21, 2014 #2


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    Gold Member

    [itex] S=\pi r^2 \Rightarrow r^2=\frac{S}{\pi} \Rightarrow r=\sqrt{\frac{S}{\pi}} [/itex]
  4. Nov 21, 2014 #3
    Thanks for the replay Shyan.

    a circel with a diameter of 1.29mm 0.6452π = 1.30 The formula you mentioned √s/π gives a value of 0.362.. ???

    Could you show me an example, I would be very thanks full
  5. Nov 21, 2014 #4


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    Careful boy! Its not [itex] r=\frac{\sqrt{S}}{\pi} [/itex], its [itex] r=\sqrt{\frac{S}{\pi}} [/itex].
  6. Nov 21, 2014 #5

  7. Nov 21, 2014 #6


    Staff: Mentor

    We don't call it "surface area" - just area. Surface area applies to surfaces in three dimensions. For objects in the plane (two dimensions), we talk about the area.
    The actual formula is ##A = \pi r^2##. If r is given in mm., then the units of area will be square millimeters or mm2. The formula shouldn't explicitly show units.
    From the equation ##A = \pi r^2##, to get the radius, solve for r. To do this, divide both sides of the equation by ##\pi##, then take the square root. Once you have the radius r, you can get the diameter using d = 2r.
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