Negative pedal curve

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Negative pedal curve

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Circle — negative pedal curve of a limaçon

In the plane, for every point X other than P there is a unique line through X perpendicular to XP. For a given curve in the plane and a given fixed point P, called the pedal point, the negative pedal curve is the envelope of the lines XP for which X lies on the given curve.

The negative pedal curve of a pedal curve with the same pedal point is the original curve.

For a parametrically defined curve, its negative pedal curve with pedal point (0; 0) is defined as

X[x,y]=\frac{(x'^2-y'^2)y'+2xyx'}{xy'-yx'}
Y[x,y]=\frac{(x'^2-y'^2)x'+2xyy'}{xy'-yx'}

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