Proving that a limit of a two-variable function does not exist

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bellerevolte
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Find the limit as (x,y) -> (0,0) of (x^4 + y^4)/(x^3 + y^3)

This was a question from a recent homework set (class homework is done online), and the server accepted 0 as an answer. However, the actual answer is that the limit does not exist. My professor told us this afterwards and proposed that we find a way to prove that the limit indeed does not exist (I'm assuming this means to find a function from which the limit does not approach 0). But every function I have tried so far ends up making the limit 0.

Anyone up for a challenge? :)
 
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