Question

Write the explicit formula for the geometric sequence.

a1 = -5 a2 = 20 a3 = -80

Answer 1

geometric is
an=(a1)(r^(n-1))
a1=first term
r=common ratio

so

a1=-5, given

a2=20 so

a2=20=-5(r^(2-1))
20=-5(r^1)
20=-5(r)
divide both sides by -5
-4=r

the formula is
an=-5(-4)^(n-1)