The first term of a geometric series is -1, and the common ratio is -3. How many terms are in the series if its sum is 182?

Question

The first term of a geometric series is -1, and the common ratio is -3.  How many terms are in the series if its sum is 182?

hartnn · Answer

did u note down the last formula i gave ??
use same to find n.

rainbow_dash · Answer

okay so 182= -1*-3^n -1/-4?

hartnn · Answer

yes,but be careful with brackets.
$182=(-1)\frac{(-3)^n-1}{-4}$
so $182*4+1=(-3^n)$
can u go ahead ?

rainbow_dash · Answer

I did it out and got -8.99, and an answer for my quiz is 9. I guess they rounded a little

hartnn · Answer

actually its exact 9
because 182*4+1=729 and 3^9=729.
but u got the whole thing,right ?

rainbow_dash · Answer

ahhh that's what i did wrong! i had a negative 728 and added one and got 727. see what i did wrong :) thanks dude!

hartnn · Answer

welcome :)