I am starting with my question with the note "Assume no math skills". Given that, all down votes are welcomed. (At the expense of better understanding of course!)

Given my first question: What is meant by the perimeter of a Sector

  1. Why is the value of $\pi$ not exactly $3$? why is it $3.14$.......... or a fraction $\frac{22}{7}$?
  2. Is the value of $\pi$ of $3.14$... or $\frac{22}{7}$ the same as for $3$ dimensions?
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2 Answers2


There are several ways to define $\pi$, but whichever you choose it is a number that happens to be irrational (it's not equal to any fraction, although some fractions are close). It does represent the ratio between a circle's circumference and diameter for any circle (but not for spheres, squares, or other shapes).

With regards to your first question, a sector is a part of a circle, like a slice of pie (the food). Its perimeter consists of a round bit on the outside, and two straight bits toward the center.

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I'm not exactly sure what you mean by the value of $ \pi $ for 3 dimensional applications. $ \pi $ is a constant value - it is always equal to $ 3.1415 \ldots $.

Of course formulas don't always translate the same from 2 dimensional to 3 dimensional. For example, the area of a circle is $ \pi r^2 $ but the volume contained within a sphere is $ \frac{4\pi}{3} r^3 $. You could almost say that $ \pi $ in 2 dimensional geometry is analogous to $ \frac{4\pi}{3} $ but then this causes fault in other places with $ \pi $, such as surface area.

Jon Claus
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    it s not true that $\pi$is always a constant, the value of $pi$ depends on number of things; for example to see how $\pi$ could be equal to 42 look at this : http://math.stackexchange.com/questions/254620/pi-in-arbitrary-metric-spaces – jimjim May 18 '13 at 22:35
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    Area of a sphere is not $\frac{4}{3}\pi r^3$. This is the volume of a ball. Area of a sphere is $4\pi r^2$. – tomasz May 18 '13 at 23:48
  • You're right, I misspoke. I forgot to switch terminology between the two cases. – Jon Claus May 19 '13 at 03:46