Write the explicit formula for the geometric sequence.

a1 = - 5 a2 = 20 a3 = - 80

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)