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圆切线在几何学习中发挥着重要的作用,下面给出几种过圆上和圆外一点做圆的切线的尺规画法,并给出相应的证明。命题:已知给定的☉O和圆上或圆外一定点A,过点A作圆☉O的切线。画法1:(1)如图1,点A在☉0上,联结OA并延长,以A为圆心,OA为半径画弧交☉A延长线于B,再分别以O、B为圆心,以大于OA长度为半径画弧交于C、D两点,联结C、D,CD即为☉O的切线。
Circle tangent in geometry learning plays an important role, the following is given several round and round the circle to do a little tangent ruler method, and give the corresponding proof. Proposition: Known given ☉O and round or round a certain point A, A point for a round ☉O tangent. Method 1: (1) As shown in Fig. 1, point A is on ☉0, connecting OA and extending, taking A as the center of circle, OA is the radius of arcs ☉A extending line at B, and then taking O and B as center of circle, To be greater than the length of OA as the radius of the arc to pay C, D two points, the link C, D, CD is 切 O tangent.