Question

Find S8 for the series -2 + -10 + -50 + -250 +…

Answer 1

A third try? IF we are to assume that it is a Geometric Series, which the problem statement does NOT say, you can just add them up.

Trivial
a = a

Boring
a + ar = a(1+r)

Now we're getting somewhere interesting!
a + ar + ar^2 = a(1 + r + r^2) = \(a\cdot\dfrac{1 - r^{3}}{1-r}\)

Generally
a + ar + ar^2 + ... + r^{n-1} = a(1 + r + r^2 + ... + r^(n-1)) = \(a\cdot\dfrac{1 - r^{n}}{1-r}\)

That's all it takes.

Determine
a = the 1st term
r = the common ratio
n = the desired sum rank

and you are on your way.

Answer 2

this is no help.. thnks anyway

Answer 3

Seriously? You're not going to do ANY work?

What's the first term? You must be able to do that.

Answer 4

Come on!! Tell me the first term. Look up at the series and read it off.

Answer 5

im kool... thnks again