有無一條equation係可能成為一個心形的graph?

In geometry, a cardioid is the curve traced by a point on the edge of a circular wheel that is rolling around a fixed wheel of the same size. The resulting curve is roughly heart-shaped[1], with a cusp at the place where the point touches the fixed wheel.
The cardioid is a roulette, and can be viewed as either an epicycloid with one cusp or as a member of the family of limaçons. It is also a type of sinusoidal spiral, and is the inverse curve of a parabola[2] with the focus as the center of inversion[3].



圖片參考：http://upload.wikimedia.org/wikipedia/commons/6/6f/EpitrochoidOn1.gif



圖片參考：http://bits.wikimedia.org/skins-1.5/common/images/magnify-clip.png
A cardioid generated by a rolling circle.


圖片參考：http://upload.wikimedia.org/wikipedia/commons/thumb/6/63/CardioidsLabeled.PNG/300px-CardioidsLabeled.PNG



圖片參考：http://bits.wikimedia.org/skins-1.5/common/images/magnify-clip.png
Four graphs of cardioids oriented in the four cardinal directions, with their respective polar equations.

2010-03-24 14:15:42 補充：
資料來源: STEVIE-G™(知識長)

http://en.wikipedia.org/wiki/Cardioid