A Limit Proof Using EstimationDate: 06/01/98 at 08:33:39 From: mostyn Subject: Limit How do I show that the limit of x^2 as x->(-2) is 4, using the delta-epsilon definition of a limit? Thanks, Mostyn Date: 06/01/98 at 11:02:37 From: Doctor Rob Subject: Re: Limit The definition says that for any epsilon > 0, no matter how small, you can find a delta > 0 such that: |x-(-2)| = |x+2| < delta implies that |x^2-4| < epsilon The idea is to start with what you want to show, that |x^2-4| < epsilon, and to manipulate this until you can get it into the form |x+2| < some expression in epsilon. Then picking delta to be this expression in epsilon will do, and the proof is to work backwards through the steps of the manipulation. In this case: |x^2-4| < epsilon <==> |(x+2)^2 - 4*(x+2)| < epsilon <== |x+2|^2 + 4*|x+2| < epsilon (by the triangle inequality) <==> |x+2|^2 + 4*|x+2| - epsilon < 0 Now you can use the Quadratic Formula to solve for |x+2|, and thus find an upper bound on |x+2| in terms of epsilon. That will be what you choose for delta. One tricky part is that each step needs an implication arrow in one direction (<==) but not necessarily in the other. Another tricky part is that there isn't necessarily a unique answer. In this case, you could have proceeded like this instead: |x^2-4| < epsilon <==> |(x+2)*(x-2)| < epsilon <==> |x+2|*|x-2| < epsilon <== |x+2|*(4 + |x+2|) < epsilon (by the triangle inequality) <== 5*|x+2| < epsilon (provided |x+2| <= 1) and so on. At the end, you pick delta to be the minimum of 1 and the expression involving epsilon, and this ensures the "provided ..." part. Other expressions for delta in terms of epsilon may also work. -Doctor Rob, The Math Forum Check out our web site! http://mathforum.org/dr.math/ |
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