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Babynini
 one year ago
A partial sum of an arithmetic sequence is given. find the sum.
1+5+9+...+401
Babynini
 one year ago
A partial sum of an arithmetic sequence is given. find the sum. 1+5+9+...+401

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anonymous
 one year ago
Best ResponseYou've already chosen the best response.0ok, we have \(a_1=1\) and common difference is \(d=4\), last term is \(a_n=401\) and you sure know that nth term of the sequence (a_n) is given by:\[a_n=a_1+(n1)d\]find \(n\)\[401=1+4(n1) \\ n1=100 \\ n=101\]and for sum use the formula\[S=\frac{n(a_1+a_n)}{2}=\frac{101(1+401)}{2}=101\times 201\]

Babynini
 one year ago
Best ResponseYou've already chosen the best response.0What if it is a geometric sum?

Babynini
 one year ago
Best ResponseYou've already chosen the best response.0For example: 1+3+9+...+2187 (thanks for the help on the other one by the way :) )

Babynini
 one year ago
Best ResponseYou've already chosen the best response.0from that we get, a=, r=3, n = ?

Babynini
 one year ago
Best ResponseYou've already chosen the best response.0Plugging some numbers here and there I get that a_8=2187 now what?

Babynini
 one year ago
Best ResponseYou've already chosen the best response.0nvm, I got it now xD it = 3280
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