A geometric sequence can always be expressed as:
a(n)=ar^(n-1), a=initial value, r=common ratio, n=term number
We are told that a(5)=25 and r=5 so we can say:
25=a5^(5-1)
25=a5^4
25=625a
a=1/25 so now we know our explicit formula for this geometric sequence:
a(n)=(1/25)(5^(n-1)) and the first three terms will be a(1), a(2), and a(3)
1/25, 1/5, 1
