Answer
$\theta\in[0,2\pi]$
(For just the upper heart, set $\theta\in[0,2\pi]$)
Work Step by Step
Work done in geogebra:
First, define r as a function of $\theta$
Use
x = $ f(\theta)\cdot\cos\theta,\qquad y=f(\theta)\cdot\sin\theta$
to plot the curve.
Change the domain until the curve retraces itself. Here,
$\theta\in[0,2\pi]$
(For just the upper heart, set $\theta\in[0,2\pi]$)
The points A, B, C show the direction in which the parameter increases.
