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英语翻译The vertical line test to determine whethera curve is th

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英语翻译
The vertical line test to determine whether
a curve is the graph of a function states:If every vertical line in
the ry-plane intersects a given curve in at most one point,then
the curve is the graph of a function.Explain why this is true.
大概意思:如果任意直线最多和一直曲线有一个交点,那么这条曲线一定会对应一个方程.解释为什么以上所诉是对的.
可以用中文回答,英语解答和最快回答优先考虑采纳,过程但一定要清楚!
英语翻译The vertical line test to determine whethera curve is th
如图:设C(x1,y1),D(x2,y2)设直线为y=ax+b,双曲线为y=k/x.联立消去y得:ax^2+bx-k=0所以x1x2=-k/a.则E(0,y1),F(x20),所以y1=k/x1.直线EF的斜率为-y1/x2=-( k/x1) /x2=-k/(x1x2)= -k/(-k/a)=az直线CD的斜率也是a.所以EF∥CD.