Sentence:  &#91;[https://books.google.com/books?id=R0d76m-Be10C&pg=PA16 page 16]&#93; The figurate number's <WSD>gnomon</WSD> is therefore 1+nb in the general case. A trivial family of numbers, those for which b=0, might be called "linear". ... &#91;[https://books.google.com/books?id=R0d76m-Be10C&pg=PA17 page 17]&#93; Clearly, a figurate number of rank n is equal to the sum of its first n <WSD>gnomons</WSD>, and in particular "the sum of the first" n "odd numbers is equal to" n^2. 
Meaning of the word marked with <WSD> tags:  A number representing the increment between two (dots in geometric figures formed of dots).