所谓三角法,就是将几何问题转化为三角问题,运用三角函数的定义,三角恒等变换及正弦定理、余弦定理等来完成几何命题证明的方法。如何将几何问题转化为三角问题并运用三角知识来证明呢?让我们通过一些具体的例子来进行分析: 例1.已知AB为☉O的弦,过A、B分别引这圆的切线交于点C。P为☉O上任意一点,自P引AB、BC、CA的垂线,垂足依次为D、E、F。求证:PD~2=PE·PF。证明:连AP、BP,并设∠PAD=α,∠PBD=β The so-called triangulation method is to convert geometric problems into trigonometric problems, using the definition of trigonometric functions, trigonometric constant transformation, sine theorem, cosine theorem, etc. to complete the method of proving geometric propositions. How to convert geometric problems into trigonometric problems and use trigonometric knowledge to prove it? Let us analyze through some concrete examples: Example 1. Know that AB is a chord of ☉O, cross A and B respectively lead the tangents of this circle. At point C. P is an arbitrary point on ☉O, since P refers to the perpendicular lines of AB, BC, and CA, and the descending order is D, E, and F. Proof: PD~2=PE·PF. Proof: Even AP, BP, set PAD=α, PBD=β