# Thread: Q in qeometric squence

1. ## Q in qeometric squence

Hi
If geometric sequence = a1 = 3 a7 = 2187 and r = 3 find S7
in this Qustion which formula we use
I use this : Sn = a1(1-r^n ) / 1- r
the answer was = - 39

2. $S_n = \frac{a(1-r^n)}{1- r}$

$S_7 = \frac{3(1-3^7)}{1- 3}= 3279$

3. If r < 1 or r > 1

also we use same formula

$
S_n = \frac{a(1-r^n)}{1- r}
$

???

4. Yes, that's true. If r= 1, then the sum is na.