3 years ago

Need help with this question!

Mathematics

1 answer:

adell [148]3 years ago

7 0

Answer:

im sorry

Step-by-step explanation:

i dont know

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PLS help I will give brainliest

Savatey [412]

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Step-by-step explanation: hope this helps pls mark me brainliest

6 0

3 years ago

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What is the sum of the geometric series in which a1 = −5, r = 3, and an = −405?

xxMikexx [17]

$a_1=-5;\ r=3;\ a_n=-405\\\\a_n=a_1r^{n-1}\\\\subtitute\\\\-5\cdot3^{n-1}=-405\ \ \ |divide\ both\ sides\ by\ (-5)\\\\3^{n-1}=81\\\\3^{n-1}=3^4\iff n-1=4\to n=5\\\\S_n=\dfrac{a_1(1-r^n)}{1-r}\\\\S_5=\dfrac{-5(1-3^5)}{1-3}=\dfrac{-5(1-243)}{-2}=\dfrac{5\cdot(-242)}{2}=5\cdot(-121)=-605\\\\\\Answer:\boxed{S_5=-605}$