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 Question from Delfina, a student: In a geometric sequence the second term is 15 and the fifth term is -405. Find the sum of the first eight terms

Hi Delfina,

The first five terms of a geometric sequence are

$a, ar, ar^2, ar^3, ar^{4}.$

The second term is 15 and the fifth term is -405. This gives you two equations in $a$ and $r.$
Solve for $a$ and $r.$

Write back if you need more assistance,
Penny

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