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.
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.
Determine the sign in the denominator. If [latex]p<0[/latex], use subtraction. If [latex]p>0[/latex], use addition.
Write the coefficient of the trigonometric function as the given eccentricity.
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].
The directrix is [latex]y=-p[/latex], so we know the trigonometric function in the denominator is sine.
Because [latex]y=-2,-2<0[/latex], so we know there is a subtraction sign in the denominator. We use the standard form of
[latex]r=\frac{ep}{1-e \sin \theta }[/latex]
and [latex]e=3[/latex] and [latex]|-2|=2=p[/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].
Because the directrix is [latex]x=p[/latex], we know the function in the denominator is cosine. Because [latex]x=4,4>0[/latex], so we know there is an addition sign in the denominator. We use the standard form of
[latex]r=\frac{ep}{1+e \cos \theta }[/latex]
and [latex]e=\frac{3}{5}[/latex] and [latex]|4|=4=p[/latex].