Homework Help: Find the Limit of Multivariable Function

1. Feb 22, 2012

Rapier

1. The problem statement, all variables and given/known data
Find the limit of:
lim (x,y) ->(0,0) (x^2*y)/(x^2 + y^4)

2. Relevant equations
x=rcosθ
y=rsinθ

3. The attempt at a solution
lim r->0 for all steps
L = (rcosθ)^2*(rsinθ)/[(rcosθ)^2 + (rsinθ)^4]
L = r^3 (cosθ)^2 (sinθ) / [ r^2 * (cosθ)^2 + r^4 * (sinθ)^4]
L = r (cos^2 * sin ) / (cos^2 + r^2*sin^4)

That's as far as I can get. I thought about trying to use r^2 in the denominator to work back around to a sin^2 + cos^2 or trying to convert the terms in the denominators into r and either cos or sin so I could get rid of one of the terms and nothing seems to be working.

Oh, I should mention that I tried a couple of different paths and my limit seemed to always equal 0. I know that I cannot prove a limit exists with this method, I can only prove that the limit does not exist. I'm fairly certain the limit = 0, but I can't figure out a way to determine it.

This is a problem that I'm supposed to be thinking about for discussion on Thursday. So I don't want the answer, but guidance. Thanks!!

-r

2. Feb 22, 2012

sunjin09

try using the fact x^2+y^4>=x^2