三角形中的射影定理、余弦定理和正弦定理,文[1]已(于1954年)推证到凸n边形。文[2]则应用不同的方法(复数方法)对文[1]的结论进行了再论证。文[3]将前两个定理推证到n面体。本文拟应用向量代数中的一个最基本的等式推证,较易得到空间n边形中的射影定理和余弦定理。 The projective theorem, the cosine theorem, and the sine theorem in triangles, [1] have been (in 1954) to the convex n-gon. The paper [2] applies a different method (complex method) to reconstruct the conclusion of [1]. In [3], the first two theorems are deduced to the n-hedron. This paper proposes to use one of the most basic equations in vector algebra to deduce that it is easier to obtain the projective and cosine theorems in the spatial n-gon.