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Circles are described by the Cartesian equation
\qquad (x-a)^2 + (y-b)^2 = r^2
where the centre is at (a,b) and the radius is r.

So, if you know the centre and radius of a circle you can write down its equation immediately.

## Summary/Background

• The equation of a circle can be written as (x-a)²+(y-b)² = r²
• This circle has centre at (a,b)
• The circle has radius r
 Circles can be displayed on your graphic calculator, for example, on the TI-83: Select the Y= screen:Enter Y1 = √(R-(X-A))+BEnter Y2 = -√(R-(X-A))+BThen select the GRAPH screen. You can then choose different values for the constants A, B and R. For example, to make R = 4, press 4 ALPHA R. You may also need to adjust the scaling to get a good display of the circle.

This page uses jsMath You can get a better display of the maths by downloading special TeX fonts from jsMath. In the meantime, we will do the best we can with the fonts you have, but it may not be pretty and some equations may not be rendered correctly.

## Glossary

### circle

a conic curve with equation (x-a)²+(y-b)²=r²

### equation

A statement that two mathematical expressions are equal.

### graph

A diagram showing a relationship between two variables.
The diagram shows a vertical y axis and a horizontal x axis.

Full Glossary List