Heptagon

[Gr. hept a, seven, and gõnia, angle.] ( Geometry.) A plane figure of seven sides. The area of a regular H. is equal to the square of one of its sides multiplied into the constant number 3-6339124. Heptag'onal, a . [Fr.) Having seven angles or sides. Heptagonal Numbers . ( Arith .) Figurate numbers of the 2d order and 5th cla ; they are formed by the succe ive addition of the terms of the arithmetical series 1, 6, 11, 16, ., whose common difference is 5. Thus the first four H. N. are, 1, 7, 18, 34, and the nth is (5 n-3).