Difference between revisions of "Circle"

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== Traditional Definition ==
 
== Traditional Definition ==
A '''circle''' is defined as the [[set]] (or [[locus]]) of [[point]]s with an equal distant from a fixed point.  The fixed point is called the [[center]] and the distance from the center to a point on the circle is called the [[radius]].
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A '''circle''' is defined as the [[set]] (or [[locus]]) of [[point]]s in a [[plane]] with an equal distance from a fixed point.  The fixed point is called the [[center]] and the distance from the center to a point on the circle is called the [[radius]].
 
<center>[[Image:circle1.PNG]]</center>
 
<center>[[Image:circle1.PNG]]</center>
  
 
== Coordinate Definition ==
 
== Coordinate Definition ==
Using the traditional definition of a circle, we can find the general form of the equation of a circle on the coordinate plane given its radius, <math> r </math>, and center <math> (h,k) </math>.  We know that each point, <math> (x,y) </math>, on the circle which we want to identify is a distance <math> r </math> from <math> (h,k) </math>.  Using the distance formula, this gives <math> \sqrt{(x-h)^2 + (y-k)^2} = r </math> which is more commonly written as
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Using the traditional definition of a circle, we can find the general form of the equation of a circle on the [[coordinate plane]] given its radius, <math> r </math>, and center <math> (h,k) </math>.  We know that each point, <math> (x,y) </math>, on the circle which we want to identify is a distance <math> r </math> from <math> (h,k) </math>.  Using the distance formula, this gives <math> \sqrt{(x-h)^2 + (y-k)^2} = r </math> which is more commonly written as
  
 
<center><math> (x-h)^2 + (y-k)^2 = r^2. </math></center>
 
<center><math> (x-h)^2 + (y-k)^2 = r^2. </math></center>
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== Area of a Circle ==
 
== Area of a Circle ==
The area of a circle is <math> \pi r^2 </math> where <math> \pi </math> is the mathematical constant [[pi]].
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The area of a circle is <math> \pi r^2 </math> where <math> \pi </math> is the mathematical constant [[pi]] and <math> r </math> is the radius.
  
 
=== Archimedes' Proof ===
 
=== Archimedes' Proof ===

Revision as of 17:35, 30 June 2006

Traditional Definition

A circle is defined as the set (or locus) of points in a plane with an equal distance from a fixed point. The fixed point is called the center and the distance from the center to a point on the circle is called the radius.

Circle1.PNG

Coordinate Definition

Using the traditional definition of a circle, we can find the general form of the equation of a circle on the coordinate plane given its radius, $r$, and center $(h,k)$. We know that each point, $(x,y)$, on the circle which we want to identify is a distance $r$ from $(h,k)$. Using the distance formula, this gives $\sqrt{(x-h)^2 + (y-k)^2} = r$ which is more commonly written as

$(x-h)^2 + (y-k)^2 = r^2.$

Example: The equation $(x-3)^2 + (y+6)^2 = 25$ represents the circle with center $(3,-6)$ and radius 5 units.

Circlecoordinate1.PNG

Area of a Circle

The area of a circle is $\pi r^2$ where $\pi$ is the mathematical constant pi and $r$ is the radius.

Archimedes' Proof

We shall explore two of the Greek mathematician Archimedes demonstrations of the area of a circle. The first is much more intuitive.

Archimedes envisioned cutting a circle up into many little wedges (think of slices of pizza). Then these wedges were placed side by side as shown below:

Pizzawedges2.PNG

As these slices are made infinitely thin, the little green arcs in the diagram will become the blue line and the figure will approach the shape of a rectangle with length $r$ and width $\pi r$ thus making its area $2\pi r$.

Archimedes also came up with a brilliant proof of the area of a circle by using the proof technique of reductio ad absurdum.

Archimedes' actual claim was that a circle with radius $r$ and circumference $C$ had an area equivalent to the area of a right triangle with base $C$ and height $r$. First let the area of the circle be $A$ and the area of the triangle be $T$. We have three cases then.

Case 1: The circle's area is greater than the triangle's area.

This proof needs to be finished.

Formulas

  • Area $\displaystyle \pi r^2$
  • circumference $\displaystyle 2\pi r$

Other Properties

  • awaiting diagrams to add stuff on inscribed angles + tangents.

Practice Problems

See Also

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