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在平面几何中,当两圆内切或外切时,我们常用作两圆公切线的方法来解决问题,而作一个圆的辅助切线依然能起到“桥梁”和“转化”的作用,得到新的证明方法.事实上,(1)在弦的端点处作圆的切线,能得到弦切角与对应的圆周角相等;(2)在半径的外端点处作圆的切线,可以得到切线与半径垂直,构造线与线之间的垂直或相应直线的平行关系;(3)如果已有圆的一条切线,再添加一条辅助线,在两条切线相交的情况下,可以利用切线长定理解
In plane geometry, when the two circles are inscribed or circumscribed, we often use it as a method of solving the problem of two common tangent lines, while making a circle auxiliary tangent still can play the role of “bridge” and “transformation” In fact, (1) making a tangent to the circle at the end of the string gives a chordal cut-off equal to the corresponding circumferential angle; (2) making a tangent to the circle at the outer end of the radius , You can get the tangent and the radius perpendicular to the construction line and the line between the vertical or the corresponding straight line parallel relationship; (3) If you have a circle of tangent, then add a guideline, the intersection of two tangent line, you can Using tangential long-term understanding