Let $P$ be any point and let $OP=r$, where $O$ is the origin and let $OP$ make an angle $\theta$ with the axis, then $r$ and $\theta$ are called Polar coordinates of the point $P$, and the coordinates may be written as $(r,\theta)$.
Find the polar equation of the circle whose Cartesian equation is $x^2+y^2=4x$
Put $x=r\cos\theta$, $y=r\sin\theta$, then
Therefore the polar equation of the circle is $=4\cos\theta$
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