Arithmetical Progression

Arithmetical Progression, a series of numbers each one of which differs from the preceding by a constant amount. Thus 2, 5, 8, 11 .... or 3, 2-1/3, 1-2/3 ...., the differences in the two cases being 3 and -2/3 respectively. The sum of such a series is the average term (the mean of first and last) multiplied by the number of terms.