Question

Find S8 for the series -2 + -10 + -50 + -250 +...
i got 781248

Answer 1

@Michele_Laino

Answer 2

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

Answer 3

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

Answer 4

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

Answer 5

no wait it shud be 195312

Answer 6

a1 should be -2 not 1

Answer 7

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

Answer 8

so did i get the right anwser?

Answer 9

it is -195312 since you are summing all negative terms

Answer 10

okay thank u

Answer 11

:)