Question and Answers Forum

All Questions      Topic List

None Questions

Previous in All Question      Next in All Question      

Previous in None      Next in None      

Question Number 185023 by aba last updated on 17/Jan/23

provet that : Σ_(k=1) ^n (1/(n+k))>ln(((2n+1)/(n+1)))

$$\mathrm{provet}\:\mathrm{that}\::\:\underset{\mathrm{k}=\mathrm{1}} {\overset{\mathrm{n}} {\sum}}\frac{\mathrm{1}}{\mathrm{n}+\mathrm{k}}>\mathrm{ln}\left(\frac{\mathrm{2n}+\mathrm{1}}{\mathrm{n}+\mathrm{1}}\right) \\ $$

Answered by witcher3 last updated on 18/Jan/23

ln(((2n+1)/(n+1)))=∫_(n+1) ^(2n+1) (1/x)dx=Σ_(k=1) ^n ∫_(n+k) ^(k+n+1) (dx/x)≤Σ_(k=1) ^n (1/(n+k))  ∀x∈[n+k,n+k+1];(1/x)≤(1/(n+k))  ⇔Σ_(k=1) ^n (1/(n+k))≥ln(((2n+1)/(n+1)))  >>

$$\mathrm{ln}\left(\frac{\mathrm{2n}+\mathrm{1}}{\mathrm{n}+\mathrm{1}}\right)=\int_{\mathrm{n}+\mathrm{1}} ^{\mathrm{2n}+\mathrm{1}} \frac{\mathrm{1}}{\mathrm{x}}\mathrm{dx}=\underset{\mathrm{k}=\mathrm{1}} {\overset{\mathrm{n}} {\sum}}\int_{\mathrm{n}+\mathrm{k}} ^{\mathrm{k}+\mathrm{n}+\mathrm{1}} \frac{\mathrm{dx}}{\mathrm{x}}\leqslant\underset{\mathrm{k}=\mathrm{1}} {\overset{\mathrm{n}} {\sum}}\frac{\mathrm{1}}{\mathrm{n}+\mathrm{k}} \\ $$ $$\forall\mathrm{x}\in\left[\mathrm{n}+\mathrm{k},\mathrm{n}+\mathrm{k}+\mathrm{1}\right];\frac{\mathrm{1}}{\mathrm{x}}\leqslant\frac{\mathrm{1}}{\mathrm{n}+\mathrm{k}} \\ $$ $$\Leftrightarrow\underset{\mathrm{k}=\mathrm{1}} {\overset{\mathrm{n}} {\sum}}\frac{\mathrm{1}}{\mathrm{n}+\mathrm{k}}\geqslant\mathrm{ln}\left(\frac{\mathrm{2n}+\mathrm{1}}{\mathrm{n}+\mathrm{1}}\right) \\ $$ $$>> \\ $$

Terms of Service

Privacy Policy

Contact: info@tinkutara.com