Conic Sections in Polar Coordinates: Learn It 3

Defining Conics in Terms of a Focus and a Directrix

So far we have been using polar equations of conics to describe and graph the curve. Now we will work in reverse; we will use information about the origin, eccentricity, and directrix to determine the polar equation.

How To: Given the focus, eccentricity, and directrix of a conic, determine the polar equation.

  1. Determine whether the directrix is horizontal or vertical. If the directrix is given in terms of [latex]y[/latex], we use the general polar form in terms of sine. If the directrix is given in terms of [latex]x[/latex], we use the general polar form in terms of cosine.
  2. Determine the sign in the denominator. If [latex]p<0[/latex], use subtraction. If [latex]p>0[/latex], use addition.
  3. Write the coefficient of the trigonometric function as the given eccentricity.
  4. Write the absolute value of [latex]p[/latex] in the numerator, and simplify the equation.
Find the polar form of the conic given a focus at the origin, [latex]e=3[/latex] and directrix [latex]y=-2[/latex].

Find the polar form of a conic given a focus at the origin, [latex]e=\frac{3}{5}[/latex], and directrix [latex]x=4[/latex].

Find the polar form of the conic given a focus at the origin, [latex]e=1[/latex], and directrix [latex]x=-1[/latex].

Convert the conic [latex]r=\frac{1}{5 - 5\sin \theta }[/latex] to rectangular form.

Convert the conic [latex]r=\frac{2}{1+2\text{ }\cos \text{ }\theta }[/latex] to rectangular form.