Monday 15 April 2013

real analysis - Limit at infinity of cubic roots and square roots without using conjugate $lim_{x to infty} frac{sqrt[3]{x+2}}{sqrt{x+3}}$




$$\lim_{x \to \infty} \frac{\sqrt[3]{x+2}}{\sqrt{x+3}} $$




How would you proceed to find this limit, by eyeballing I would guess it foes to zero since the numerator has a smaller power than the denominator, normaly I would use the binomial theorem if I had something like $$\lim_{x \to \infty} \frac{\sqrt[3]{x+2}-1}{\sqrt{x+3}-1} $$ But here I don't know how to find the limit since I can't really use the binomial theorem.


Answer




If you factorize you get
$$\frac{x^{1/3}(1+2/x)^{1/3}}{x^{1/2}(1+3/x)^{1/2}} = \frac{(1+2/x)^{1/3}}{x^{1/6}(1+3/x)^{1/2}} $$
I'll let you do the limit yourself.


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