# What is the limit of `(x^3+y^3)/(x^2+y^2)` as (x,y)-->(0,0)? I'm supposed to use polar coordinates to find the limit. The assignment says: "If (`r,theta` ) are polar coordinates of the...

What is the limit of `(x^3+y^3)/(x^2+y^2)` as (x,y)-->(0,0)?

I'm supposed to use polar coordinates to find the limit.

The assignment says: "If (`r,theta` ) are polar coordinates of the point (x,y) with r > 0 (or equal to 0), note that r --> 0 as (x,y) --> (0,0)."

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### 2 Answers

The problem requests to use parametric forms, hence, you need to convert the Cartesian coordinates x,y into polar coordinates, thus, you need to substitute `r*cos t` for x and `r*sin t` for y, such that:

`lim_(x,y -> 0) (x^3 + y^3)/(x^2 + y^2) = lim_(r->0) (r^3*cos^3 t + r^3*sin^3 t)/(r^2*cos^2 t + r^2*sin^2 t)`

Factoring out `r^3` and `r^2` yields:

`lim_(r->0) (r^3*(cos^3 t+sin^3 t))/(r^2*(cos^2 t+sin^2 t))`

Using the basic formula of trigonometry, `cos^2 t+sin^2 t = 1` , yields:

`lim_(r->0) r*(cos^3 t+sin^3 t) = 0`

**Hence, evaluating the given multivariable limit, using parametric forms, yields `lim_(x,y -> 0) (x^3 + y^3)/(x^2 + y^2) = 0. ` **

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### User Comments

Thanks a lot for your answer!