引言在解决两边及其一边对角求第三边问题中,经常使用正弦定理,或余弦定理,但是发现虽然使用余弦定理简便,计算量小,但是也会出现多解情况,有时多解成立,有时又要取舍,导致最后不敢确信求出来的结果是对还是错,最终放弃该方法,拿到题目就是正弦定理,相比之下正弦定理计算量比较大,容易出现计算上的失误,对于基础差的同学往往是直接放弃,不愿继续算下去,所以我们急需明确余弦定理该怎么用,与正弦定理相比它的优势在哪,两种方法如何取长补短. INTRODUCTION In solving the problem of seeking the third side diagonally on both sides and one side, the sine or cosine theorem is often used. However, it is found that although the cosine theorem is simple and the computational complexity is small, there are many solutions and sometimes multiple solutions are established. And sometimes have to choose, resulting in the final not convinced that the results obtained is right or wrong, and ultimately give up the method, get the title is the sine theory, compared with sinusoidal theorem calculation is relatively large, prone to computational errors, for Basically poor students are often directly give up, do not want to continue to count, so we urgently need to clear cosine theorem how to use, compared with the sine theory, its advantages, where the two methods learn from each other.