What is the sum of the geometric series in which a1 = 7, r = 3, and an = 1,701?

Hint: Sn= a1(1-rn)/1-r, r ≠ 1, where a1 is the first term and r is the common ratio.

Question

What is the sum of the geometric series in which a1 = 7, r = 3, and an = 1,701?

Hint: Sn= a1(1-rn)/1-r, r ≠ 1, where a1 is the first term and r is the common ratio.

callie_love_me · Answer

That formula basically gives it out. Just plug the numbers into the Sn formula.

mhchen · Answer

So you have the geometric series formula:
$$S_{n} = a_{1} * \frac{ 1-r^{n} }{ 1-r }$$

a1 = 7
r = 4

I hope you know an = a1 * r^n so
an = 7 * (4^n)
an/7 = 4^n
$$\log _{4}(1701) = n$$

And plug a1, r, and n into that formula at the top.