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平面内一点Ｐ（ｘ０，ｙ０）到直线Ａｘ＋Ｂｙ＋Ｃ＝０的距离ｄ＝｜Ａｘ０＋Ｂｙ０＋Ｃ｜Ａ２＋Ｂ２．利用这一公式去研究某些问题时，常常需要对“Ａｘ０＋Ｂｙ０＋Ｃ”的符号作出判断．因高中数学教材对上述符号的判断未作说明，所以笔者特推证归纳下列三种简便的判定方...
The distance from the point P(x0,y0) to the line Ax+By+C=0 in the plane d=|Ax0+By0+C|A2+B2. When using this formula to study certain problems, it is often necessary to make a judgment on the sign of “Ax0+By0+C”. Because the high school mathematics textbooks do not explain the above symbols, the author extols the following three simple criteria: