Conic Sections and Circles

Conic Sections and Circles

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 signsHyperbola
\(A\) and \(B\) have the same signEllipse
\(A=B\)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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