Translation invariant valuations in the space of planes

The paper is related to the problem of measure generation in the space IE of planes in IR ³ by combinatorial, translation invariant valuations. General results concerning that problem have been derived in 1994 and 1996 (this journal vol. 31, no. 4. and vol. 33, no. 4, respectively). The purpose of the present article is to give a proof of two geometrical identities on which the theorem on valuations in the space IE can be based. The article consists of the motivational first part that contains the basic concepts from the theory of combinatorial valuations and measure generation in IE, and the second that gives a proof of the identities in question.