# Evaluate ##lim_{(x,y)\to(0,0)\frac{xy}{y-x^3}}##

• duarthiago
In summary, the limit of f(x,y) as (x,y) approaches (0,0) does not exist because the sign of the denominator changes no matter how close the point is to the origin. This can be seen by considering the line y = delta - x for 0 < x < delta, which passes through the origin and causes the denominator to become negative as x approaches delta. Therefore, the limit of f does not exist at the origin.
duarthiago

## Homework Statement

Evaluate ##lim_{(x,y)\to(0,0)\frac{xy}{y-x^3}}##

## The Attempt at a Solution

I've tried move towards the origin along the path ##\gamma_{1} (t) = (t,2t^3)##, which leads to ##2t=0##. Then I've tried to use ##\gamma_{2} (t) = (0,t)## which leads to ##\frac{0}{t}##, then applying the L'Hospital's rule this limit is 0 too (can I always do it when I have a single-variable limit?).

I've suspected that the limit exists and its value is 0, then I've tried to use polar coordinates:

##lim_{(x,y)\to(0,0)\frac{xy}{y-x^3}}##

##lim_{r \to 0^{+}} \frac{r^2 cos(k)sin(k)}{r(sin(k)-r^2 cos^3 (k))}##

But I've got stuck there because I can't figure how to show that ##\frac{cos(k)sin(k)}{sin(k)-r^2 cos^3 (k)}## is bounded.A further question: It is related to the question on L'Hospital's rule, above. In regards to the limit

##lim_{(x,y) \to (0,0)} \frac{xy(x-y)}{x^4 + y^4}##
The path ##\gamma_{A} (t) = (t,0)## leads to 0.
##\gamma_{B} (t) = (t,2t)## leads to ##\frac{-2t^3}{17t^4}=\frac{-2}{17t}##. Can I tend t to zero, conclude that the limit goes to the minus infinity and because of that say that the original limit doesn't exist? Maybe it seems a silly question, but until now I haven't seen something like this happening. It is always something bounded multiplied by zero.

Well, I worked a little bit more on the problem and polar coordinates wasn't necessary. Taking a curve like ##\gamma (t) = (\sqrt[3]{t-t^2},t)## is enough.
About my second question: now is quite clear that if I choose a curve that goes through the origin, then I have to study what happens when t -> 0 to conclude something about the behavior of the original function when (x,y) -> (0,0).

duarthiago said:

## Homework Statement

Evaluate ##lim_{(x,y)\to(0,0)\frac{xy}{y-x^3}}##

## The Attempt at a Solution

I've tried move towards the origin along the path ##\gamma_{1} (t) = (t,2t^3)##, which leads to ##2t=0##. Then I've tried to use ##\gamma_{2} (t) = (0,t)## which leads to ##\frac{0}{t}##, then applying the L'Hospital's rule this limit is 0 too (can I always do it when I have a single-variable limit?).

You don't need to resort to L'Hopital's theorem when the numerator is constantly zero: the limit will be zero.

I've suspected that the limit exists and its value is 0, then I've tried to use polar coordinates:

The obvious problem here - which you do not mention - is that $f(x,y) = \frac{xy}{y - x^3}$ is not defined on the curve $y = x^3$, which of course passes through the origin.

##lim_{(x,y)\to(0,0)\frac{xy}{y-x^3}}##

##lim_{r \to 0^{+}} \frac{r^2 cos(k)sin(k)}{r(sin(k)-r^2 cos^3 (k))}##But I've got stuck there because I can't figure how to show that ##\frac{cos(k)sin(k)}{sin(k)-r^2 cos^3 (k)}## is bounded.

It isn't: For each $r$, it is unbounded when $\sin k = r^2 \cos^3 k$. But what you actually want is the limit of $r$ times that expression. That might be bounded as $r \to 0$, even if $\frac{cos(k)sin(k)}{sin(k)-r^2 cos^3 (k)}$ is not. But even that would not be sufficient to conclude that the limit of $f$ exists: it only considers what happens as you approach along straight lines, and there are other ways of approaching the origin.

Exercise: Set $\delta > 0$ and consider the line $y = \delta - x$ for $0 < x < \delta$. This lies inside the open ball $\|(x,y)\| < \delta$. Look at $f(x,\delta - x)$. What happens to the sign of the denominator between $x = 0$ and $x = \delta$?

What can you then conclude about the existence of the limit of $f$ at the origin?

A further question: It is related to the question on L'Hospital's rule, above. In regards to the limit

##lim_{(x,y) \to (0,0)} \frac{xy(x-y)}{x^4 + y^4}##
The path ##\gamma_{A} (t) = (t,0)## leads to 0.
##\gamma_{B} (t) = (t,2t)## leads to ##\frac{-2t^3}{17t^4}=\frac{-2}{17t}##. Can I tend t to zero, conclude that the limit goes to the minus infinity and because of that say that the original limit doesn't exist?

Yes.

Thank you for your insightful answer. About the exercise, I'm not sure, but it seems that ##y## goes through (0,##\delta##) and (##\delta##,0). Then ##x^3## (which also lies on the circle) will be greater than ##\delta - x## at some point, so the sign of the denominator becomes negative. ##y## won't pass at the origin, but the sign will always change no matter how small delta is. So, can I conclude the limit of ##f## at origin doesn't exist precisely because of that behavior?

## What does "lim" mean in this equation?

"lim" stands for limit, which is a mathematical concept that represents the behavior of a function as its input approaches a certain value or point.

## Why is the limit being evaluated at (0,0)?

In this equation, (0,0) represents the point at which both the x and y values are equal to 0. This specific point is chosen to analyze the behavior of the function as it approaches the origin.

## How do I evaluate this limit?

To evaluate this limit, you can substitute the values of x and y as they approach 0 into the function and see what value it approaches. Alternatively, you can use algebraic techniques such as factoring or simplifying the expression to find the limit.

## What does the result of this limit tell us about the function?

The result of the limit helps us understand the behavior of the function at the point (0,0). It can tell us if the function is continuous or discontinuous at that point, and whether the function approaches a finite value or infinity.

## What are some real-life applications of evaluating limits?

Evaluating limits is an essential concept in calculus and is used in various fields such as physics, engineering, economics, and statistics. For example, in physics, limits are used to calculate the velocity and acceleration of a moving object, while in economics, limits can help determine the marginal cost and revenue of a product.

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