# Conic Sections/Circle

The circle is the simplest and best known conic section. As a conic section, the circle is the intersection of a plane perpendicular to the cone's axis.

The geometric definition of a circle is the locus of all points a constant distance from a point and forming the **circumference** (C). The distance is the **radius** (R) of the circle, and the point is the circle's **center** also spelled as **centre**. The **diameter** (D) is twice the length of the radius.

## Equations

[edit | edit source]#### Standard Form

[edit | edit source]The standard equation for a circle with center and radius is

The radius must be greater than 0. If the radius is zero, the graph is a single point. This is a degenerate case. |

In the simplest case of a circle whose center is at the origin, the equation is simply a restatement of the *Pythagorean Theorem*:

#### General form

[edit | edit source]The general form of a circle equation is

<-g,-f> is the center of the circle.

#### Polar Coordinates

[edit | edit source]In the case of a circle centered at the origin, the polar equation of a circle is very simple because polar coordinates are essentially based on circles. For a circle with radius ,

In the more complicated case of a circle with an arbitrary location, the equation is

where is the distance from the circle's center to the origin and is the angle pointing to the circle.

There are many cases that allow the equation to be simplified. If a point on the circle is touching the origin, its polar equation may consist of a single trig function.

.....

#### Parametric Equations

[edit | edit source]When the circle's equation is parametrized with respect to , the equation becomes

.

## Example

[edit | edit source]Find the center and the radius of the following circle:
x^{2}+y^{2}+8x-10y+20=0
find by:

x^{2}+y^{2}+8x-10y+20=0

x^{2}+y^{2}+8x-10y= - 20

(x^{2}+8x)+(y^{2}-10y)= - 20
__+16__ __+25__ __+16+25__

(x^{2}+8x+16)+(y^{2}-10y+25)=21

(x+4)^{2}+(y-5)^{2}=21

Thus:

C(-4,5) radius=