find the sum of the first 5 terms of the geometric series -2,-6,-18...

Question

whpalmer4 · Answer

Are you able to find the next two terms?

Hint: a geometric sequence means that the ratio between any two adjacent terms is always the same value.  

-6/-2 = 
-18/-6 = 
4th term / -18 = 
5th term / 4th term =

johnweldon1993 · Answer

$$S_n = a \frac{ 1-r^n }{ 1-r }$$

n = the number of terms you want the sum of
a = the first term in the sequence
r = the common ratio

The common ratio looks like 3 right? -2 X 3 = -6...and -6 X 3 = -18   so yes 3 is the common ratio "r"

n Would be 5 here since you want the sum of the first 5 numbers

a would be -2 , the first term of the sequence

So

$$S_5 = -2 \frac{ 1-(3)^5 }{ 1-3 }$$

can you solve this?

anonymous · Answer

no im lost