Oblong numbers are the numbers of dots that can be placed in rows and columns in a rectangular array, each row containing one more dot than each column. The first few oblong numbers are 2, 6, 12, 20, and 30. This series of numbers is the successive sums of the series of even numbers or the products of two consecutive numbers: 2 = 1·2; 6 = 2·3 = 2 + 4; 12 = 3·4 = 2 + 4 + 6;…

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(Moch. Lutfianto)