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yuan de qie xian chang xiang deng de ni ming ti yan ge bi tu qu xian shi yuan de yi ge chong yao tiao jian ji qi chu deng zheng ming
Author(s): 
Pages: 20-21
Year: Issue:  6
Journal: ZHONGXUE SHUXUE ZAZHI(GAOZHONG BAN)

Abstract:  在同一平面上,过圆外任一点引已知圆的切线,共有两条,且切线长相等.这个命题的逆命题该怎样表述?是否成立呢?显然我们应该从平面上一个闭曲线出发,而且要求过曲线外任意一点都可以引曲线的两条切线,我们将这样的曲线称为严格闭凸曲线.这样,"圆的切线长相等"的逆命题就变成"对于平面上的给定一严格闭凸曲线,若过曲线外任一点作曲线的两条切线,有切线长相等,那么该严格闭凸曲线是圆".这是一个看似无从下手的问题,...
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