What is the polar equation of a horizontal line?

The polar equation of ##y=k## is ##r=kcsctheta##.

This can be obtained by using ##y=rsintheta##, one of the conversion equations that relate the Cartesian and polar systems. By substitution, we have ##rsintheta=k##, and consequently ##r=k/sintheta##, which is equivalent to ##r=kcsctheta##.

For a specific example, you may want to see the Youtube video, offered by Mathispower4u, at