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初中《几何》第三册关于圆的切线的判定定理为:经过半径的外端并且垂直于这条半径的直线是圆的切线。根据这个定理,要判定某条直线是圆的切线,必须满足两个条件:①直线经过半径的外端点;②该直线必须垂直于这条半径,两者缺一不可,在实际问题中,常用不同方法处理以下两类问题。一、已知某直线与圆有一公共点,求证该直线为圆的切线。说明已知条件已满足切线判定定理中的①,只要证明②成立。方法1 常连结过该点的半径,证该直线与所连半径垂直。
The judgment theorem of the tangent of the circle in the third book “Geometry” of junior high school is: The straight line passing through the outer end of the radius and perpendicular to this radius is the tangent of the circle. According to this theorem, to determine whether a straight line is a tangent of a circle, two conditions must be satisfied: 1 The line passes the outer end of the radius; 2 The line must be perpendicular to this radius, both are indispensable, in practical problems, commonly used Different methods deal with the following two types of problems. First, it is known that a certain straight line and circle have a common point, and verify that the straight line is a tangent of a circle. Explain that the known condition has satisfied 1 in the tangent decision theorem, as long as the proof 2 holds. Method 1 often links the radius of the point, verifying that the line is perpendicular to the connected radius.