Question

What is the sum of the geometric sequence -3,18,-108,.. if there are 7 terms?

Answer 1

Answer:

$S_{7}$ = - 119973

Step-by-step explanation:

The sum to n terms of a geometric sequence is

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

where a is the first term and r the common ratio

Here a = - 3 and r = 18 ÷ - 3 = - 6 , thus

 $S_{7}$   = $\frac{-3(-279936-1)}{-6-1}$

    = $\frac{-3(-279937)}{-7}$

    = $\frac{839811}{-7}$

     = - 119973