Why is Area of a Circle Equal to Pi * (Radius Squared)?Date: 07/05/2005 at 07:45:01 From: Sandra Subject: why does pi r square give the area I know the formula for the area of a circle is A = pi*r^2, but why does that give the area? Date: 07/05/2005 at 08:52:52 From: Doctor Jerry Subject: Re: why does pi r square give the area Hello Sandra, One answer is "Because one can prove that the area of a circle of radius r is pi*r^2, using calculus." There are other ways, not of proving that the area of a circle of radius r is pi*r^2, but of persuading others that this is the right formula. Kepler gave a nice, persuasive argument. It depends upon your accepting that the circumference of a circle of radius r is 2*pi*r. See the diagram at Divide the region K enclosed by a circle by rays from the origin to points of the circle, doing the division so that there are n congruent sectors. The central angle of each sector is 2*pi/n. Now imagine that we cut the region R along the radii forming the boundaries of the sectors, but do not cut into the circle itself. We then imagine "unrolling" this pie along its edge, so that the edge becomes a straight line and the sectors are upright, as on the left in the figure. Each sector is very much like a triangle. The area of one of these triangles is one-half times its base times its height: (1/2)*(2*pi*r/n)*(r) Because there are n "triangles," their total area, which is the same as area of the circle, is n*(1/2)*(2*pi*r/n)*(r), which is pi*r^2. - Doctor Jerry, The Math Forum http://mathforum.org/dr.math/ |
Search the Dr. Math Library: |
[Privacy Policy] [Terms of Use]
Ask Dr. Math^{TM}
© 1994- The Math Forum at NCTM. All rights reserved.
http://mathforum.org/dr.math/