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epi·cy·cloid /ˌɛpəˈsaɪˌklɔɪd/

外擺線

Ep·i·cy·cloid n. Geom. A curve traced by a point in the circumference of a circle which rolls on the convex side of a fixed circle.

Note: ☞ Any point rigidly connected with the rolling circle, but not in its circumference, traces a curve called an __epitrochoid__. The curve traced by a point in the circumference of the rolling circle when it rolls on the concave side of a fixed circle is called a __hypocycloid__; the curve traced by a point rigidly connected with the rolling circle in this case, but not its circumference, is called a __hypotrochoid__. All the curves mentioned above belong to the class class called roulettes or trochoids. See Trochoid.

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From: WordNet (r) 2.0

epicycloid

n : a line generated by a point on a circle rolling around

another circle