Hello,
First we must find the ratio:
\(r= \dfrac{ u_{2}}{ u_{1}} = \dfrac{20}{-5} =-4\\\\
r= \dfrac{ u_{3}}{ u_{2}} = \dfrac{-80}{20} =-4\\\\
u_{1}= -5\\
u_{2}= -5*(-4)\\
u_{3}= -5*(-4)^2\\
u_{4}= -5*(-4)^3\\
\boxed{u_{n}= -5*(-4)^{n-1}\\}\\\\
Answer\ B\)
