We can consider using right-angled triangles to prove cosine theorem, and we can divide two right-angled triangles by the height of the original triangle to prove cosine theorem. However, the methods provided in textbooks are proved by vectors.

As shown in the figure, BD = CCCOSB, AH=csinB, in the triangle AHC, there is AC? = huh? +HC？ Can prove the cosine theorem.