Let's do some calculus! (27)

Calculus Level 3

lim x x x + x 3 x + x 3 x + x 3 x + = ? \Large \lim_{x \to \infty} \ \frac{x}{x + \frac{\sqrt[3]x}{ x + \frac{\sqrt[3]x}{x + \frac{\sqrt[3]x}{x+ \cdots}}}} ~ = ~ ?


For more problems on calculus, click here .


The answer is 1.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

1 solution

Chew-Seong Cheong
Sep 26, 2016

Let the limit be L L and y = x + x 3 x + x 3 x + x 3 x + . . . y = x + \dfrac {\sqrt[3]x}{x + \frac {\sqrt[3]x}{x + \frac {\sqrt[3]x}{x +...}}} ; L = lim x x y \implies L = \lim_{x \to \infty} \dfrac xy .

Now consider y y as follows.

y = x + x 3 x + x 3 x + x 3 x + . . . = x + x 3 y \begin{aligned} y & = x + \frac {\sqrt[3]x}{x + \frac {\sqrt[3]x}{x + \frac {\sqrt[3]x}{x +...}}} = x + \frac {\sqrt[3]x}y \end{aligned}

y 2 x y x 3 = 0 Solving the quadratic of y \begin{aligned} \implies y^2 -xy - \sqrt[3] x & = 0 & \small \color{#3D99F6}{\text{Solving the quadratic of }y} \end{aligned}

y = x + x 2 + 4 x 3 2 Note that y > 0 \begin{aligned} \implies y & = \frac {x + \sqrt{x^2+4\sqrt[3]x}}2 & \small \color{#3D99F6}{\text{Note that }y > 0} \end{aligned}

Now, we have:

L = lim x x y = lim x x x + x 2 + 4 x 3 2 = lim x 2 x x + x 2 + 4 x 3 Divide up and down by x = lim x 2 1 + 1 + 4 x 5 / 3 = 2 1 + 1 + 0 = 1 \begin{aligned} L & = \lim_{x \to \infty} \frac xy = \lim_{x \to \infty} \frac x{\frac {x + \sqrt{x^2+4\sqrt[3]x}}2} \\ & = \lim_{x \to \infty} \frac {2x}{x + \sqrt{x^2+4\sqrt[3]x}} & \small \color{#3D99F6}{\text{Divide up and down by }x} \\ & = \lim_{x \to \infty} \frac 2{1 + \sqrt{1+\frac 4{x^{5/3}}}} \\ & = \frac 2{1+\sqrt {1+0}} = \boxed{1} \end{aligned}

Why you say y>0?

Andrea Bruno - 4 years, 8 months ago

Log in to reply

Because x > 0 x>0 .

Chew-Seong Cheong - 4 years, 8 months ago

It was an oral question. Just watch and answer.

Devkant Chouhan - 4 years, 7 months ago

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...