Is the statement always true?

Algebra Level 2

True or False?

For all positive integers n > 1 n>1 , the inequality below is correct. x = n + 1 2 n 1 x > 13 24 \sum_{x=n+1}^{2n} \frac{1}{x} > \frac{13}{24}

Yes No

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1 solution

Chris Lewis
Dec 11, 2020

Let s n = n + 1 2 n 1 x s_n=\sum_{n+1}^{2n} \frac{1}{x}

Then s 1 = 1 2 s_1=\frac12 and for all n > 1 n>1 , s n 1 = n 2 n 2 1 x s_{n-1}=\sum_n^{2n-2} \frac{1}{x}

so that s n s n 1 = 1 2 n 1 + 1 2 n 1 n = 1 2 n 1 1 2 n s_n-s_{n-1}=\frac{1}{2n-1}+\frac{1}{2n}-\frac{1}{n}=\frac{1}{2n-1}-\frac{1}{2n}

Since 1 2 n 1 1 2 n > 0 \frac{1}{2n-1}-\frac{1}{2n}>0 for all n > 1 n>1 , the sequence s n s_n is increasing; since s 2 = 7 12 = 14 24 > 13 24 s_2=\frac{7}{12}=\frac{14}{24}>\frac{13}{24} , the statement is always true .


Bonus question: what happens to s n s_n as n n \to \infty ?

The question as written doesn't make sense. "For every n n , we can find an n n ..."? And then there's a formula that involves n n ? This is a confusing abuse of notation. It seems like it's just supposed to mean "For every n > 1 n > 1 this following is true..."

Richard Desper - 6 months ago

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Thanks. I've updated the problem statement to reflect this.

In the future, if you have concerns about a problem's wording/clarity/etc., you can report the problem. See how here .

Brilliant Mathematics Staff - 6 months ago

Hi Chris! I would very much appreciate it if you could check on our previous discussion. I hope this doesn't bother you! :)

Inquisitor Math - 6 months ago

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