Difference between revisions of "Circle"

(Introductory)
m
Line 1: Line 1:
 
A '''circle''' is a geometric figure commonly used in Euclidean [[geometry]].
 
A '''circle''' is a geometric figure commonly used in Euclidean [[geometry]].
 
{{asy image|<asy>unitsize(2cm);draw(unitcircle,blue);</asy>|right|A basic circle.}}
 
{{asy image|<asy>unitsize(2cm);draw(unitcircle,blue);</asy>|right|A basic circle.}}
 +
 
== Traditional Definition ==
 
== Traditional Definition ==
 
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]].  
 
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]].  
Line 6: Line 7:
  
 
== 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
+
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
 
+
<cmath>(x - h)^2 + (y - k)^2 = r^2.</cmath>
<center><math> (x-h)^2 + (y-k)^2 = r^2 </math></center>
 
  
'''Example:''' The equation <math> (x-3)^2 + (y+6)^2 = 25 </math> represents the circle with center <math> (3,-6) </math> and radius 5 units.
+
'''Example:''' The equation <math>(x - 3)^2 + (y + 6)^2 = 25</math> represents the circle with center <math>(3,-6)</math> and radius 5 units.
 
<center>[[Image:Circlecoordinate1.PNG]]</center>
 
<center>[[Image:Circlecoordinate1.PNG]]</center>
  
 
== 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]] and <math> r </math> is the radius.
+
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 05:26, 8 August 2014

A circle is a geometric figure commonly used in Euclidean geometry.

[asy]unitsize(2cm);draw(unitcircle,blue);[/asy]

Enlarge.png
A basic circle.

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.

The radius and center of a circle.

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 $\pi r^2$.

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.

Case 2: The triangle's area is greater than the circle's area.

Case 3: The circle's area is equal to the triangle's area.

Assume that $A>T$. Let $P$ be the area of a regular polygon that is closest to the circle's area. Therefore we have $A-P<A-T$ so $P>T$. Let the apothem be $a$ and the perimeter be $p$ so the area of a regular polygon is one half of the product of the perimeter and apothem. The perimeter is less than the circumference so $p<2\pi r$ and the apothem is less than the radius so $a<r$. Therefore $P=\frac{1}{2}ap<\frac{1}{2}r\cdot 2\pi r=T$. However it cannot be both $P>T$ and $P<T$. So $A\not >T$.

Template:Incomplete

Proof Using Calculus

Let the circle in question be $x^2 + y^2 = r^2$, where r is the circle's radius. By symmetry, the circle's area is four times the area in the first quadrant. The area in the first quadrant can be computed using a definite integral from 0 to r of the function $f(x) = \sqrt(r^2 - x^2)$. Using the substitution $x = r \sin u, dx = r \cos u$ gives the indefinite integral as $\frac{r^2}{2} (u - \frac{\sin 2u}{2}) + C$, so the definite integral equals $\frac{r^2}{2} * \frac{\pi}{2}$. Multiplying by four gives the area of the circle as $\pi r^2$.

Related Formulae

Other Properties and Definitions

[asy]draw(unitcircle);draw((-0.8,1)--(1,1),Arrow);draw((1,1)--(-0.8,1),Arrow);draw((0,1)--(1,0));[/asy]

Enlarge.png
A circle with a tangent and a chord marked.
  • A line that touches a circle at only one point is called the tangent of that circle. Note that any point on a circle can have only one tangent.
  • A line segment that has endpoints on the circle is called the chord of the circle. If the chord is extended to a line, that line is called a secant of the circle.
  • The measure of an inscribed angle is always half the measure of the central angle with the same endpoints.
  • Chords, secants, and tangents have the following properties:
    • The perpendicular bisector of a chord is always a diameter of the circle.
    • The perpendicular line through the tangent where it touches the circle is a diameter of the circle.
    • The Power of a point theorem.

Other interesting properties are:

  • A right triangle inscribed in a circle has a hypotenuse that is a diameter of the circle.
  • Any angle formed by the two endpoints of a diameter of the circle and a third distinct point on the circle as the vertex is a right angle.

Problems

Introductory

  • What is the area of a circle with radius $3?$
  • Under what constraints is the circumference (in inches) of a circle greater than its area (in square inches)?

Intermediate

\[\mathrm{(A) \ } 13\qquad\mathrm{(B) \ } \frac{44}{3}\qquad\mathrm{(C) \ } \sqrt{221}\qquad\mathrm{(D) \ } \sqrt{255}\qquad\mathrm{(E) \ } \frac{55}{3}\qquad\]

(Source)

  • Let

\[S_1=\{(x,y)|\log_{10}(1+x^2+y^2)\le 1+\log_{10}(x+y)\}\]

and

\[S_2=\{(x,y)|\log_{10}(2+x^2+y^2)\le 2+\log_{10}(x+y)\}\]. What is the ratio of the area of $S_2$ to the area of $S_1$?

\[\mathrm{(A) \ } 98\qquad \mathrm{(B) \ } 99\qquad \mathrm{(C) \ } 100\qquad \mathrm{(D) \ } 101\qquad \mathrm{(E) \ }  102\]

(Source)

Olympiad

(<url>viewtopic.php?=217167 Source</url>)

See Also