Question

what is the explicit formula 0.5,-0.1,0.02,-0.004,0.0008

Answer 1

-0.1 / 0.5 = -0.2
0.02 / -0.1 = -0.2  

common ratio = -0.2

Geometric Sequence

an = a1 r^(n-1)

an  = 0.5 * -0.2^(n-1)