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Calculus Level 1

Evaluate the limit lim x + x 2 3 x 3 + 1 3 \displaystyle \lim_{x \rightarrow +\infty} \dfrac{\sqrt{x^{2}-3}}{\sqrt[3]{x^{3}+1}} .


The answer is 1.

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2 solutions

Hobart Pao
Sep 22, 2015

The standard method is to divide each term by the highest power term in the denominator. In this case, it's x. We don't have to bother with absolute values because x is approaching positive infinity in this case. lim x x 2 3 x x 3 + 1 3 x \lim_{x \rightarrow \infty} \dfrac{\dfrac{\sqrt{x^{2}-3}}{x}}{\dfrac{\sqrt[3]{x^{3}+1}}{x}} = lim x x 2 3 x 2 x 3 + 1 3 x 3 3 =\lim_{x \rightarrow \infty} \dfrac{\dfrac{\sqrt{x^{2}-3}}{\sqrt{x^{2}}}}{\dfrac{\sqrt[3]{x^{3}+1}}{\sqrt[3]{x^{3}}}} = lim x x 2 x 2 3 x 2 x 3 x 3 + 1 x 3 3 =\lim_{x \rightarrow \infty} \dfrac{\sqrt{\dfrac{x^{2}}{x^{2}}-\dfrac{3}{x^{2}}}}{\sqrt[3]{\dfrac{x^{3}}{x^{3}}+\dfrac{1}{x^{3}}}} Cancel, and apply the rule that lim x k x n = 0 \displaystyle \lim_{x \rightarrow \infty} \dfrac{k}{x^{n}}=0 , where k is a constant. You're left with lim x 1 1 \displaystyle \lim_{x \rightarrow \infty} \dfrac{1}{1} which is simply 1 \boxed{1}

Kay Xspre
Sep 20, 2015

You may transform the function into ( x 2 3 ) 3 ( x 3 + 1 ) 2 6 = x 6 9 x 4 + 27 x 2 27 x 6 + 2 x 3 + 1 6 \sqrt[6]\frac{(x^2-3)^3}{(x^3+1)^2} = \sqrt[6]\frac{x^6-9x^4+27x^2-27}{x^6+2x^3+1} Evaluating limit will give 1 6 = 1 \sqrt[6]1 = 1

Thanks for your solution! However, there's an even easier way, i think.

Hobart Pao - 5 years, 8 months ago

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L'hopital is likely to be the last method if I encounter to strange function, but as this is a polynomial, I decided to take one instead.

Kay Xspre - 5 years, 8 months ago

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You don't need L'Hopital's here, you can either apply end behavior by just looking at it, or use the standard method I just posted.

Hobart Pao - 5 years, 8 months ago

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