一、利用图形的轴对称或中心对称特性,简化命题的分析和证明。例1 如图,设△ABC为等腰三角形BC为底边,D为从A到BC的垂线的垂足,以AD为直径作圆,由B、C依次作圆的切线BE和CF(不同于BC),E、F为切点。求证:EF弦在△ABC内部一段的长等于它在外部两段长之和。 First, the use of graphics axis symmetry or central symmetry, simplifying the analysis and proof of the proposition. Example 1 As shown in the figure, △ ABC for the isosceles triangle BC as the bottom edge, D from A to BC vertical foot of the foot, with AD as the diameter of the circle, from B, C, followed by the tangent of the tangent BE and CF ( Unlike BC), E, ​​F are tangent points. Prove: EF string △ ABC within the length of a paragraph is equal to the length of the two in the outer sum.