A conic section is a curve obtained by intersecting a plane with the surface of a cone. The three main types are the parabola, ellipse and hyperbola; a circle is a special case of an ellipse.
Parabola
Circle
Ellipse
Hyperbola
General equation for a circle, ellipse or hyperbola
\[Ax^2+By^2+Cx+Dy+E=0\]
Compare the coefficient of \(x^2\), \(A\), with the coefficient of \(y^2\), \(B\).
| Comparison of \(A\) and \(B\) | Conic classification |
|---|---|
| \(A\) and \(B\) have different signs | Hyperbola |
| \(A\) and \(B\) have the same sign | Ellipse |
| \(A=B\) | Circle (a special ellipse) |
Circle
Expanding \((x-a)^2+(y-b)^2=r^2\) gives \(x^2+y^2+dx+ey+f=0\). Completing the square reverses the conversion.
This is a circle with centre \((a,b)\) and radius \(r\).
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