割線是指與曲線至少交於兩相異點的直線。當這兩個點不斷靠近,並重合為一個點時,這條直線就變成了這條曲線的切線。
割線與正割函數的關係
在原點處建立單位圓,在處畫單位圓的切線。從原點畫一條與軸夾角為的割線,只要不等於90度,割線與切線交於點。則的正割等於原點到割線與切線交點的距離,即。
與圓的割線相關的幾何定理
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