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A hypotrochoid is a roulette traced by a point attached to a circle of radius rolling around the inside of a fixed circle of radius , where is a distance from the center of the interior circle. The parametric equations for a hypotrochoid are
(1)
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(2)
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A polar equation can be derived by computing
(3)
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(4)
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Here, the parameter is not the polar angle but is related to it by
(5)
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To get cusps in the hypotrochoid, , because then rotations of bring the point on the edge back to its starting position.
Special cases of the hypotrochoid are summarized in the table below.
curve | special values |
ellipse | |
hypocycloid | |
rose curve |
The arc length, curvature, and tangential angle are
(6)
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(7)
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(8)
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where is an elliptic integral of the second kind.