切线
Multi tool use
切線（英语：tangent line），為一幾何名詞，應用於曲線及平面圓時意義有所不同。
设L为一条曲线，A, B为此曲线上的点，过此二点作曲线的割线，令B趋向A，如果割线的极限存在，则称此极限（一条直线）为曲线在A的切线。
曲线切线和法线的定义
几何上，切线指的是一条刚好触碰到曲线上某一点的直线。更准确地说，当切线经过曲线上的某点（即切点）时，切线的方向与曲线上该点的方向是相同的，此时，“切线在切点附近的部分”最接近“曲线在切点附近的部分”（无限逼近思想）。tangent在拉丁语中就是“to touch”的意思。类似的概念也可以推广到平面相切等概念中。
P和Q是曲线C上邻近的两点，P是定点，当Q点沿着曲线C无限地接近P点时，割线PQ的极限位置PT叫做曲线C在点P的切线，P点叫做切点；经过切点P并且垂直于切线PT的直线PN叫做曲线C在点P的法线（无限逼近的思想）。
注意：平面几何中，将和圆只有一个公共交点的直线叫做圆的切线．这种定义不适用于一般的曲线；PT是曲线C在点P的切线，但它和曲线C还有另外一个交点；相反，直线l尽管和曲线C只有一个交点，但它却不是曲线C的切线。
圆的切线
与一个圆只有一个交点的直线，叫此圆的切线。
性質：
圓心與切點的連線必垂直過此切點的切線。
- 若一直線過圓上一點且垂直於過此點的半徑，則此直線為該圓的切線。
弦切角與交錯弓形內角相等。
几何学术语
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| 點 |
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直線和曲線
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- 線段
- 射線
- 直線
- 切線
- 法线
- 曲線
- 圓錐曲線
- 双曲线
- 抛物线
- 螺線
- 邊
- 周界
- 弦
- 弧
- 垂直平分線
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平面圖形 |
- 圓
- 橢圓
- 扇形
- 弓形
- 多邊形
- 三角形
- 四邊形
- 五边形
- 六边形
- 梯形
- 平行四邊形
- 菱形
- 矩形
- 正方形
- 鷂形
- 卵形线
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| 立體圖形 |
- 多面體
- 正多面體
- 四面体
- 長方體
- 立方體
- 平行六面体
- 棱柱
- 反棱柱
- 棱錐
- 圓柱體
- 圓錐
- 圓台
- 橢球
- 球體
- 球缺
- 球冠
- 球台
- 二次曲面
- 抛物面
- 雙曲面
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| 圖形關係 |
- 相似
- 全等
- 对称
- 平行
- 垂直
- 相交
- 相切
- 镜像
- 旋转
- 反演
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三角形關係 |
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| 量 |
- 距離
- 長度
- 周长
- 弧长
- 高度
- 面積
- 表面積
- 體積
- 容積
- 角度
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| 作圖 |
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| 理論 |
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