The following question is about specific geometric progression
Find a_4 and S_4 for
100, −10, 1, .
(Round your answer for S4 to three decimal places.)
The _ represents subscript.
a = 100
r = -1/20
An = ar^(n-1)
Sn = a (1-r^n)/(1-r)
A4 = 100(-1/10)^3 = 100(-1/1000) = -1/10
S4 = 100(1-(-1/10)^4)/(1 - (-1/10)) = 100(9999/10000)/(11/10) = 909/10