证明一条直线是圆的切线,常见添加辅助线的方法有(一)若知直线过圆上某一点,则作出过该点的半径(1)直接利用题目中的已知的垂直条件证明;(2)如果没有明显的垂直条件,则还需用创造垂直的条件,通常是封闭直线上的圆周角;(二)若直线与圆的公共点没有确定(1)连结圆心和直线上的点,证明此点也在该圆上;(2)过圆心作该直线的垂线,再证 To prove that a straight line is a tangent of a circle, common methods for adding an auxiliary line are: (1) If the line is known to have crossed a point on the circle, the radius (1) over which the point was made is directly verified by the known vertical condition in the title; ( 2) If there is no obvious vertical condition, it is also necessary to create a vertical condition, usually a closed circle on the perimeter; (b) if the common point of the line and the circle is not defined (1) to connect the center of the circle and the point on the line, Proof that this point is also on the circle; (2) To make a vertical line for the vertical line, retest