4.5, −22.5, 112.5, −562.5, …



Since all those give the same answer, this is a geometric sequence with

first term = a1 = 4.5 and common ratio r = -5.

To get a formula for any term, we use the general formula for the nth term
of a geometric sequence, use this



To get a formula for the sum of the first n terms, use this:




Edwin