HEPTAGON

(Yunanca köken — orijinale bakınız.) lu Geometry, a plane figure of seven sides. The area of a regular heptagon is equal to the square of one of its sides multiplied into the constant number 3-6339124, or seven fourths of the tangent of the angle at the base to radius 1. HEPTAGONAL NUMBERS, are a kind of polygonal numbers, of which the difference of the terms of the corresponding arithmetical progre ion is 5. Thus, Arithmetic als 1, G, 11, 16, 21, .; Heptagon als 1, 7, 18, 34, 55, .; the latter being formed by the contmnal addition of the terms of the first. Among other properties of heptagonal numbers there is one very remarkable; namely, that if any heptagonal number is multiplied by 40, and 9 added to the product, the sum is a square number. [s. 563]