Posted by Embraced on .
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?

Alg2 
Reiny,
plug in all your info into the formula
( you have extra information)
S(n) = a(r^n 1)/(r1)
781 = a(5^5  1)/(51)
781 = a(3124/4)
781 = 781a
a = 1
check: t(5) = 1(5^4) = 625