# Key Concepts & Glossary

## Key Concepts

- Any conic may be determined by a single focus, the corresponding eccentricity, and the directrix. We can also define a conic in terms of a fixed point, the focus [latex]P\left(r,\theta \right)[/latex] at the pole, and a line, the directrix, which is perpendicular to the polar axis.
- A conic is the set of all points [latex]e=\frac{PF}{PD}[/latex], where eccentricity [latex]e[/latex] is a positive real number. Each conic may be written in terms of its polar equation.
- The polar equations of conics can be graphed.
- Conics can be defined in terms of a focus, a directrix, and eccentricity.
- We can use the identities [latex]r=\sqrt{{x}^{2}+{y}^{2}},x=r\text{ }\cos \text{ }\theta [/latex], and [latex]y=r\text{ }\sin \text{ }\theta [/latex] to convert the equation for a conic from polar to rectangular form.

## Glossary

- eccentricity
- the ratio of the distances from a point [latex]P[/latex] on the graph to the focus [latex]F[/latex] and to the directrix [latex]D[/latex] represented by [latex]e=\frac{PF}{PD}[/latex], where [latex]e[/latex] is a positive real number

- polar equation
- an equation of a curve in polar coordinates [latex]r[/latex] and [latex]\theta [/latex]

## Licenses & Attributions

### CC licensed content, Specific attribution

- Precalculus.
**Provided by:**OpenStax**Authored by:**OpenStax College.**Located at:**https://cnx.org/contents/[email protected]:1/Preface.**License:**CC BY: Attribution.