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-   -   How do I prove this multivariable limit problem? (http://mymathforum.com/calculus/43636-how-do-i-prove-multivariable-limit-problem.html)

RedBarchetta May 9th, 2014 09:01 PM

How do I prove this multivariable limit problem?
 
Compute $\displaystyle \lim_{x,y\to (0,0)}$ $\displaystyle 3xy\over x^2+y^3$

along lines of the form $\displaystyle y=mx$, for $\displaystyle m \neq 0$. What can you conclude?

When I do it, I end up with 3m which is a limit. I know the function is discontinuous at (0,0) but I don't know how to prove it.

v8archie May 9th, 2014 09:26 PM

Quote:

Originally Posted by RedBarchetta (Post 193454)
I end up with 3m

The thing about that answer is that you get a different limit for every direction of approach to (0, 0). When when dealing with a single-variable limit, that means that the limit doesn't exist (or is a pair of one-sided limits).

Actually, I get $\frac{3m}{1 + m^2}$ (valid for all $m$) by that approach, but the problem I identified before still applies.


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