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bitangent
In mathematics, a bitangent to a curve ''C'' is a line ''L'' that touches ''C'' in two distinct points ''P'' and ''Q'' and that has the same direction as ''C'' at these points. That is, ''L'' is a tangent line at ''P'' and at ''Q''.
==Bitangents of algebraic curves==
In general, an algebraic curve will have infinitely many secant lines, but only finitely many bitangents.
Bézout's theorem implies that a plane curve with a bitangent must have degree at least 4. The case of the 28 bitangents of a quartic was a celebrated piece of geometry of the nineteenth century, a relationship being shown to the 27 lines on the cubic surface.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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