for all these types of triangles，we have only one formula to calculate its area，whereas we have learned that all different types of quadrilateral，have different formula for their areas，isn't this little unfair for triangles。
海伦公式: (S为三角形面积，abc为三角形三边，p=。添加图片注释，不超过 140 字。切割线定理从圆外一点引圆的切线和割线，切线长是这点到割线与圆交点的两条线段长的比例中项添加图片注释，不超过 140 字。
