a, 3a,...
The first term in the sequence above is a, and each term after the first is 3 times the preceding term. If the sum of the first 5 terms is 605, what is the value of a? I got 25 but the correct answer is 5. Why? THanks in advance
a, 3a,...
The first term in the sequence above is a, and each term after the first is 3 times the preceding term. If the sum of the first 5 terms is 605, what is the value of a? I got 25 but the correct answer is 5. Why? THanks in advance
So I guess you weren't far from the solution.
It would be nice if you showed it after that, so that one can tell you what was wrong.
$\displaystyle a+3a+3^2a+3^3a+3^4a=605$
This can be factored : $\displaystyle a(1+3+3^2+3^3+3^4)=605$
What's in brackets is a geometric sum. We know that $\displaystyle 1+r+r^2+\dots+r^{n-1}=\frac{1-r^n}{1-r}$
Can you do it ?