When the sequence of numbers (or a1, a2, a3,...) increases or decreases by a fixed quantity, then the sequence is in arithmetic progression (A.P.). The fixed quantity is called as common difference. For an AP, we define its first term as a and the common difference as d. The general expression for an AP is: a, a + d, a + 2d, a + 3d,.... If Tr represents the gernal term of an AP, then where In an AP, the difference of any two consecutive terms is d and is given by: