What is 1^infinity?Date: 12/10/98 at 00:39:32 From: Jonathan Hiscock Subject: What is 1^infinity? My calculus class recently came across this problem while working on l'Hopital's rule for limits. Often using a direct substitution method, we would get an answer with 1^infinity. We were told that this is an indeterminate form and to try to rewrite the problem to get an appropriate form. Our calculus teacher showed us a proof using a problem similar to: f(x) = lim 1^x x->infinity And then by using natural logs to evaluate. This method did not seem to work to me or the rest of our class. Our reasoning is that 1 = 1 * 1 = 1 * 1 * 1 = ..., and so the answer will always be one. How can I prove that 1^infinity is one or indeterminate? Thank you for your time, Jonathan Hiscock Date: 12/10/98 at 03:15:05 From: Doctor Schwa Subject: Re: What is 1^infinity? When you have something like "infinity," you have to realize that it's not a number. Usually what you mean is some kind of limiting process. So if you have "1^infinity" what you really have is some kind of limit: the base isn't really 1, but is getting closer and closer to 1 perhaps while the exponent is getting bigger and bigger, like maybe (x+1)^(1/x) as x->0+. The question is, which is happening faster, the base getting close to 1 or the exponent getting big? To find out, let's call: L = lim x->0 of (x+1)^(1/x) Then: ln L = lim x->0 of (1/x) ln (x+1) = lim x->0 of ln(x+1) / x So what's that? As x->0 it's of 0/0 form, so take the derivative of the top and bottom. Then we get lim x->0 of 1/(x+1) / 1, which = 1. So ln L = 1, and L = e. Cool! Is it really true? Try plugging in a big value of x. Or recognize this limit as a variation of the definition of e. Either way, it's true. The limit is of the 1^infinity form, but in this case it's e, not 1. Try repeating the work with (2/x) in the exponent, or with (1/x^2), or with 1/(sqrt(x)), and see how that changes the answer. That's why we call it indeterminate - all those different versions of the limit approach 1^infinity, but the final answer could be any number, such as 1, or infinity, or undefined. You need to do more work to determine the answer, so 1^infinity by itself is not determined yet. In other words, 1 is just one of the answers of 1^infinity. I hope that helps clear things up! - Doctor Schwa, The Math Forum http://mathforum.org/dr.math/ |
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