①“直線(xiàn)”表示過(guò)點(diǎn)且平行于x軸的直線(xiàn);
②若點(diǎn)P和點(diǎn)關(guān)于y軸對(duì)稱(chēng),點(diǎn)和點(diǎn)關(guān)于直線(xiàn)l對(duì)稱(chēng),則稱(chēng)點(diǎn)是點(diǎn)P關(guān)于直線(xiàn)l的二次對(duì)稱(chēng)點(diǎn).
③若圖形T關(guān)于y軸對(duì)稱(chēng)的圖形為 , 圖形關(guān)于直線(xiàn)l的對(duì)稱(chēng)圖形為 , 則稱(chēng)是圖形T關(guān)于直線(xiàn)l的二次對(duì)稱(chēng)圖形.
例如:點(diǎn)關(guān)于直線(xiàn)的二次對(duì)稱(chēng)點(diǎn)是.已知四點(diǎn).