## Precalculus (6th Edition) Blitzer

$r=\frac{3}{\sin \theta }$.
Rectangular equation is $y=3$ …… (I) The relation between polar coordinates and rectangular coordinate $y$ can be expressed as below: $y=r\sin \theta$ …… (II) Substitute values of $x\$ from (II) in (I) to get \begin{align} & y=3 \\ & r\sin \theta =3 \\ & r=\frac{3}{\sin \theta } \end{align} Hence the obtained polar expression is $r=\frac{3}{\sin \theta }$.