Explanation:
a, ar, ar², ar³, ar⁴, ar⁵
6th term = ar⁵ = ²/₂₇
1st term = a = 18
=> ar⁵ = 18r⁵ = ²/₂₇r⁵ =
-2/18 x 27
=
-1/9 x 27


r⁵ =
-1/243



√r⁵ = 5√^-1/₂₄₃
= -1/3