Question

Write the sum using summation notation assuming the suggested pattern continues 9-18+36-72

Answer 1

I think it is 45 sorry if its wrong

Answer 2

Let's first work out the pattern. Since it's an geometric progression, we can find the sum to n terms of the pattern.

$S_n = \frac{a(r^n - 1)}{r - 1}$
$S_n = \frac{9((-2)^n - 1)}{-3}$
$S_n = -3((-2)^n - 1)$

Now that we've developed our geometric progression, we can start to add sigma notation.

k
∑(-3[(-2)^n - 1])
n = 1