Write an explicit formula for the arithmetic sequence whose common difference is -18

Let the first term be a(0).  Then the formula for a(n) is a(0)-18(n-1).

Check:  What's the first term?  Let n = 1:  a(1) = a(0)-18(1-1) = a(0) (correct)

What's the second term?    Let -n =2:  a(2) = a(0)-18(2-1) = a(0) - 18

and so on.

a variable is any characteristics, number, or quantity that can be measured or counted. a variable may also be called a data item. hope i !