Answer:
[tex]S_{7}[/tex] = - 119973
Step-by-step explanation:
The sum to n terms of a geometric sequence is
[tex]S_{n}[/tex] = [tex]\frac{a(r^n-1)}{r-1}[/tex]
where a is the first term and r the common ratio
Here a = - 3 and r = 18 ÷ - 3 = - 6 , thus
[tex]S_{7}[/tex] = [tex]\frac{-3(-279936-1)}{-6-1}[/tex]
= [tex]\frac{-3(-279937)}{-7}[/tex]
= [tex]\frac{839811}{-7}[/tex]
= - 119973
