The sum of a geometric series whose first three terms are 8000, -12000, and 18000 is 57875. What is the last term of the series?
+36916 r = -1.5
a1 = 8000
Sum of a series === a1 (1-rn)/(1-r)
57875 = 8000 (1- (-1.5)n )/(1- -1.5) now solve for n , the number of terms
57875 = 3200 (1-(-1.5)n )
= 3200 - 3200(-1.5n)
54675 = -3200 (-1.5n)
-17.0859375 = (-1.5)n
n=7 to find the 7 th term x7 = a1 (rn-1) = 8000 (-1.5)6 = 91125
