Question and Answers Forum

All Questions      Topic List

Limits Questions

Previous in All Question      Next in All Question      

Previous in Limits      Next in Limits      

Question Number 114239 by bemath last updated on 18/Sep/20

$$\underset{x\to \mathrm{\infty }}{\lim }\frac{\sqrt[3]{\cos \left(\frac{4}{x}\right)}-1}{\cos \left(\frac{2}{x}\right)-\cos \left(\frac{4}{x}\right)}?$$

Commented by bemath last updated on 18/Sep/20

$$\underset{x\to \mathrm{\infty }}{\lim }\frac{\sqrt[3]{1-\frac{8}{x^2}}-1}{(1-\frac{2}{x^2})-(1-\frac{8}{x^2})}=$$$$\underset{x\to \mathrm{\infty }}{\lim }\frac{(1-\frac{8}{3x^2})-1}{\left(\frac{6}{x^2}\right)}=-\frac{8}{3.6}=-\frac{4}{9}$$

Answered by Dwaipayan Shikari last updated on 18/Sep/20

$$\underset{x\to \mathrm{\infty }}{\lim }\frac{\sqrt[3]{1-\frac{1}{2}{\left(\frac{4}{x}\right)}^2}-1}{2sin\frac{3}{x}sin\frac{2}{x}}=\frac{1-\frac{8}{3x^2}-1}{2.\frac{6}{x^2}}=-\frac{4}{9}$$

Answered by mathmax by abdo last updated on 19/Sep/20

$$\mathrm{let}\mathrm{f}\left(\mathrm{x}\right)=\frac{{\left(\cos \left(\frac{4}{\mathrm{x}}\right)\right)}^{\frac{1}{3}}-1}{\cos \left(\frac{2}{\mathrm{x}}\right)-\cos \left(\frac{4}{\mathrm{x}}\right)}\mathrm{here}\mathrm{we}\mathrm{do}\mathrm{the}\mathrm{changement}\frac{1}{\mathrm{x}}=\mathrm{t}\Rightarrow$$$$\mathrm{f}\left(\mathrm{x}\right)=\mathrm{f}\left(\frac{1}{\mathrm{t}}\right)=\frac{{\left(\cos \left(4\mathrm{t}\right)\right)}^{\frac{1}{3}}-1}{\cos \left(2\mathrm{t}\right)-\cos \left(4\mathrm{t}\right)}(\mathrm{x}\to \mathrm{\infty }\Rightarrow \mathrm{t}\to 0)\Rightarrow$$$$\cos \left(4\mathrm{t}\right)\sim 1-{8\mathrm{t}}^2,\cos \left(2\mathrm{t}\right)\sim 1-{2\mathrm{t}}^2\Rightarrow$$$$\mathrm{f}\left(\frac{1}{\mathrm{t}}\right)\sim \frac{{(1-{8\mathrm{t}}^2)}^{\frac{1}{3}}-1}{1-{2\mathrm{t}}^2-1+{8\mathrm{t}}^2}\sim \frac{1-\frac{8}{3}{\mathrm{t}}^2-1}{{6\mathrm{t}}^2}=-\frac{8}{18}=-\frac{4}{9}\Rightarrow$$$${\lim }_{\mathrm{x}\to +\mathrm{\infty }}\mathrm{f}\left(\mathrm{x}\right)=-\frac{4}{9}$$

Terms of Service

Privacy Policy

Contact: info@tinkutara.com