**SEQUENCE:**

A sequence is an arrangement of numbers in a definite order and according to some rule.

**Example:** 1, 3, 5,7,9, … is a sequence where each successive item is 2 greater than the preceding term and 1, 4, 9, 16, 25, … is a sequence where each term is the square of successive natural numbers.

**TERMS :**

The various numbers occurring in a sequence are called ‘terms’. Since the order of a sequence is fixed, therefore the terms are known by the position they occupy in the sequence.

**Example:** If the sequence is defined as

**ARITHMETIC PROGRESSION (A.P.):**

An Arithmetic progression is a special case of a sequence, where the difference between a term and its preceding term is always constant, known as common difference, i.e., d. The arithmetic progression is abbreviated as A.P.

The general form of an A.P. is

∴ a, a + d, a + 2d,… For example, 1, 9, 11, 13.., Here the common difference is 2. Hence it is an A.P.

In an A.P. with first term a and common difference d, the nth term (or the general term) is given

by .

a_{n} = a + (n – 1)d.

…where [a = first term, d = common difference, n = term number

**Example:** To find seventh term put n = 7

∴ a_{7} = a + (7 – 1)d or a_{7} = a + 6d

The sum of the first n terms of an A.P. is given by

S_{n} = [2a + (n – 1)d] or [a + 1]

where, 1 is the last term of the finite AP.

If a, b, c are in A.P. then b = and b is called the arithmetic mean of a and c.