我们知道，若一条直线与圆有唯一公共点，则这条直线叫做圆的切线，课本给出切线的两个判定定理：定理1若圆心到一条直线的距离等于圆的半径，则这条直线是圆的切线．定理2经过半径的外端并且垂直于这条半径的直线是圆的切线．定理2与定理1的明显区别是定理2明确指出直线过圆上 We know that if a line and a circle have a unique common point, the line is called a tangent to the circle, and the textbook gives two decision theorems for the tangent: Theorem 1 If the distance from the center to a line is equal to the radius of the circle, then this line It is a round tangent. Theorem 2 is a tangent to a circle that passes through the outer end of the radius and is perpendicular to this radius. The obvious difference between Theorem 2 and Theorem 1 is that Theorem 2 explicitly states that the straight line is round