Answer: Choice C) 11 + 13 + 15 + 17 + ...
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Explanation:
We're adding terms in the form (2n+5)
The first term is when n = 3, so
2n+5 = 2*3+5 = 6+5 = 11
The next term is when n = 4, so
2n+5 = 2*4+5 = 8+5 = 13
The next term is when n = 5, so
2n+5 = 2*5+5 = 10+5 = 15
And so on
This means,
[tex]\displaystyle \sum_{n=3}^{\infty} (2n+5) = 11+13+15+17+\ldots[/tex]
which matches with choice C. The three dots indicates the pattern goes on forever.
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Choice B does not have these dots, so that's why choice B is not the answer. If we replaced the infinity symbol with 7, then choice B would be the answer.
I.e,
[tex]\displaystyle \sum_{n=3}^{7} (2n+5) = 11+13+15+17+19[/tex]
