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)."
2 Answers | Add Yours
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. `
Thanks a lot for your answer!
We’ve answered 319,186 questions. We can answer yours, too.Ask a question