Sentence:  Furthermore , the middle number of the Lo Shu is not only the physical mean between every opposing pair of the other numbers , by reason of its central position ; it is also their mathematical mean , since it <WSD>is</WSD> equal to half the sum of every opposing pair , all of which equal 10 . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)