od apples » Nedelja, 19. Jul 2015, 18:08
[dispmath]\lim_{n\to\infty}\,n^2\Bigg(e^{\frac{1}{n^2}}-\cos^{\sqrt2}\left(\frac{1}{n}\right)\Bigg)=L[/dispmath]
Resenje:
Ovaj zadatak je najlakse raditi preko razvoja:
[dispmath]e^x=1+x+\frac{x^2}{2!}+\cdots+\frac{x^n}{n!}+o\left(x^n\right)\;(x\to0)[/dispmath][dispmath]\cos(x)=1-\frac{x^2}{2!}+\frac{x^4}{4!}-\cdots+(-1)\frac{x^{2n}}{(2n)!}+o\left(x^{2n}\right)\;(x\to0)[/dispmath][dispmath](x+1)^m=1+mx+\frac{m(m-1)x^2}{2!}+\cdots+\frac{m(m-1)\cdots(m-n+1)}{n!}+o\left(x^n\right)\;(x\to0)[/dispmath]
[inlmath]\frac{1}{n}[/inlmath] i [inlmath]\frac{1}{n^2}[/inlmath] mozemo da koristimo u prethodnim "formulama" umesto [inlmath]x[/inlmath], posto [inlmath]\frac{1}{n}\to0\;(n\to\infty)[/inlmath] i [inlmath]\frac{1}{n^2}\to0\;(n\to\infty)[/inlmath]. Iz prethodnog imamo:
[dispmath]e^{\frac{1}{n^2}}=1+\frac{1}{n^2}+o\left(\frac{1}{n^2}\right)\;(n\to\infty)[/dispmath][dispmath]\cos\left(\frac{1}{n}\right)=1-\frac{1}{2n^2}+o\left(\frac{1}{n^2}\right)\;(n\to\infty)[/dispmath][dispmath]\cos^{\sqrt2}\left(\frac{1}{n}\right)=\left(1-\frac{1}{2n^2}\right)^{\sqrt2}=1-\frac{\sqrt2}{2n^2}+o\left(\frac{1}{n^2}\right)\;(n\to\infty)[/dispmath]
Konacno,
[dispmath]L=\lim_{n\to\infty}\,n^2\left(1+\frac{1}{n^2}+o\left(\frac{1}{n^2}\right)-\Bigg(1-\frac{\sqrt2}{2n^2}+o\left(\frac{1}{n^2}\right)\Bigg)\right)[/dispmath][dispmath]L=\lim_{n\to\infty}\,n^2\left(\frac{1}{n^2}+\frac{\sqrt2}{2n^2}\right)=\lim_{n\to\infty}\,\left(1+\frac{\sqrt2}{2}\right)=1+\frac{\sqrt2}{2}[/dispmath]
Poslednji put menjao
Daniel dana Nedelja, 19. Jul 2015, 20:16, izmenjena samo jedanput
Razlog: Ispravljene greške x→∞ u n→∞