Question

Write the explicit formula for the geometric sequence.

a1 = -5 a2 = 20 a3 = -80

Answer 1

if its geometric, then youll need to determine the common ratio

Answer 2

in general:

a1 = a1
a2 = a1*r
a3 = a2*r , or a1*r*r, or a1*r^2
a4 = a3*r, or a1*r^3
a5 = a1*r^4
...
an = a1*r^(n-1)

Answer 3

notice that since a2 = a1*r

we can algebra this to: r = a2/a1

Answer 4

-5x(-4)^n-1