The Geometric Sequence Formula:
[tex]a_n=a_1r^{n-1}[/tex]
We have:
[tex]a_1=-2,\ a_2=8,\ a_3=-32,\ a_4=128,\ ...[/tex]
[tex]d=\dfrac{a_{n+1}}{a_n}\to d=\dfrac{8}{-2}=-4[/tex]
Substitute:
[tex]a_n=-22\cdot(-4)^{n-1}=-22\cdot(-4)^n\cdot(-4)^{-1}=-22\cdot(-4)^n\cdot\left(-\dfrac{1}{4}\right)=5.5(-4)^n[/tex]
