Given:
The geometric sequence is
[tex]4, -12, 36,...[/tex]
To find:
The sum of first 8 terms of the given geometric sequence.
Solution:
We have,
[tex]4, -12, 36,...[/tex]
Here, the first term is 4 and the common ratio is
[tex]r=\dfrac{-12}{4}[/tex]
[tex]r=-3[/tex]
The sum of first n terms of a geometric sequence is
[tex]S_n=\dfrac{a(r^n-1)}{r-1}[/tex]
Where, a is the first term and r is the common ratio.
Putting n=8, a=4 and r=-3, we get
[tex]S_8=\dfrac{4((-3)^8-1)}{-3-1}[/tex]
[tex]S_8=\dfrac{4(6561-1)}{-4}[/tex]
[tex]S_8=-6560[/tex]
Therefore, the sum of first 8 terms is -6560.
