# Alg2

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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 -

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

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