If the sum of a geometric series is 781, the number of terms is 5, the common ratio is 5 and the last term is 625, what is the first term a1?

Answer 1

plug in all your info into the formula
( you have extra information)

S(n) = a(r^n -1)/(r-1)
781 = a(5^5 - 1)/(5-1)
781 = a(3124/4)
781 = 781a
a = 1

check: t(5) = 1(5^4) = 625

Answer 2

if the sum of eight terms of a geometric series is
781/640 and the common ratio is 3/4, what is the fourth term of the series?