Question

Geometric sequence help

Answer 1

Hello,

First we must find the ratio:

$r= \dfrac{ u_{2}}{ u_{1}} = \dfrac{20}{-5} =-4\\\\ r= \dfrac{ u_{3}}{ u_{2}} = \dfrac{-80}{20} =-4\\\\ u_{1}= -5\\ u_{2}= -5*(-4)\\ u_{3}= -5*(-4)^2\\ u_{4}= -5*(-4)^3\\ \boxed{u_{n}= -5*(-4)^{n-1}\\}\\\\ Answer\ B$