Quantities are said to be in arithmetic progressions (A.P.) when they increase or decrease by a common difference.

E.g. I. 3, 7, 11, 15…..

II. 8, 2, -4, -10…..

III. a, a+d, a+2d, a+3d…..

**Common difference: **The common difference (d) is found
by subtracting any term
of the series from the next term.

E.g. in I. d = 7-3 = 4

In II. d = 2-4 = -6

In III. d = a+d – d = a

- The nth term of an arithmetic progression is given by:

**T _{n }= a + (n-1) d**

Where a is the first term of the series, d is the common difference and n is the number of terms.

- If n be the number of terms, and if L denotes the last term or nth term than

**L = a + (n-1) d**

*Basic Questions*