The cosine theorem is used in solving triangulation problems and in physics when solving problems of addition of unidirectional oscillations. However, this theorem is used only for the analytical calculation of triangles or when solving problems of adding two oscillations. Here we propose a generalization of the cosine theorem for the case of calculating polygons of arbitrary shape and consider its application for calculating the results of adding an arbitrary number of unidirectional oscillations or coherent waves with an arbitrary phase distribution.