## anonymous one year ago Find S8 for the series -2 + -10 + -50 + -250 +... i got 781248

1. anonymous

@Michele_Laino

2. Michele_Laino

it is a geometric sequence whose constant is q=5 and first term is a1=-2

3. anonymous

okay. and i plug it to the formula and got 781248

4. Michele_Laino

we have to apply this formula: $\Large {S_8} = {a_1}\frac{{1 - {q^8}}}{{1 - q}} = 1 \cdot \frac{{1 - {5^8}}}{{1 - 5}} = ...?$

5. anonymous

no wait it shud be 195312

6. anonymous

a1 should be -2 not 1

7. Michele_Laino

oops.. $\Large {S_8} = {a_1}\frac{{1 - {q^8}}}{{1 - q}} = \left( { - 2} \right) \cdot \frac{{1 - {5^8}}}{{1 - 5}} = ...?$

8. anonymous

so did i get the right anwser?

9. Michele_Laino

it is -195312 since you are summing all negative terms

10. anonymous

okay thank u

11. Michele_Laino

:)