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 please help me ..
Follow Math Help Forum on Facebook and Google+
$\displaystyle S_n = \frac{a(1-r^n)}{1- r} $ $\displaystyle S_7 = \frac{3(1-3^7)}{1- 3}= 3279 $
If r < 1 or r > 1 also we use same formula $\displaystyle S_n = \frac{a(1-r^n)}{1- r} $ ???
Yes, that's true. If r= 1, then the sum is na.
View Tag Cloud