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