If a sequence of values follows a pattern of adding a fixed amount from one term to the next, it is referred to as an arithmetic sequence. The number added to each term is constant (always the same).The fixed amount is called the common difference, d,referring to the fact that the difference between two successive terms yields the constant value that was added. To find the common difference, subtract the first term from the second term.
find the 8th term of arithmetic sequence 3,6,9,12,........
by using a formula to find the d=a2-a1
then the common difference or d is equal to 3