余弦定理有多种证明方法,统编教材采用的是通过“坐标”的证明方法。这里,再介绍另外两种证法。一、用勾股定理证明在ΔABC中,作BQ⊥AC,在直角ΔABQ中,根据勾股定理,得 C~2=AQ~2+QB~2∵ AQ=b-acosC Cosine theorem has a variety of methods of proof, the use of textbooks is through the “coordinate” method of proof. Here, introduce two other kinds of law. First, with the Pythagorean theorem proves that in ΔABC, as BQ⊥AC, at right angles ΔABQ, according to the Pythagorean theorem, C ~ 2 = AQ ~ 2 + QB ~ 2∵ AQ = b-acosC