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割线是指与曲线至少交于两相异点的直线。当这两个点不断靠近,并重合为一个点时,这条直线就变成了这条曲线的切线。
在原点处建立单位圆,在处画单位圆的切线。从原点画一条与轴夹角为的割线,只要不等于90度,割线与切线交于点。则的正割等于原点到割线与切线交点的距离,即。
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