判断三角形的形状是一类重要的问题 ,通常使用三角变换 (含正弦、余弦定理 )求解 ,也可用向量形式给出 .本文给出△ABC为正三角形的一组充要条件 ,供学习参考 ,也可从中充分体会数学的对称美 .条件 1　a =b =c .条件 2　A =B =C .条件 1 ,条件 2是正三角形的定义 ,是判断正三角 Judging the shape of a triangle is an important class of problems. It is usually solved using a trigonometric transformation (including the sine and cosine theorem), and it can also be given in a vector form. This article gives a set of necessary and sufficient conditions for △ABC to be a regular triangle for learning reference. The symmetric beauty of mathematics can also be fully appreciated. Condition 1 a =b =c. Condition 2 A =B =C. Condition 1, Condition 2 is the definition of a regular triangle, which is the judgment of the positive triangle