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I am very confused about the following:

whenever I put in into wolfram alpha the answer it gives me is "indeterminate", is it not possible to simplify fractional exponents or something? if the exponent is 2/6 can i simplify it to 1/3 and get -1 as the answer? Provided I'm working with real numbers.

What I put in wolfram alpha was "(-1)^(1/3) real numbers" to which it answered "-1", then when I tried "(-1)^(2/6) real numbers" it answered "indeterminate".

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  • $\begingroup$ If you square first you get all the $6^{th}$ roots of $1$ including $1$. $\endgroup$
    – Mark Bennet
    Oct 18, 2014 at 17:45
  • $\begingroup$ See math.stackexchange.com/questions/956541/what-is-8-frac23 $\endgroup$
    – lab bhattacharjee
    Oct 18, 2014 at 17:48
  • $\begingroup$ @labbhattacharjee ok, so, then why does wolfram say "indeterminate" for (-1)^(2/6) and "-1" for (-1)^(1/3)? Keep in mind that I'm talking about real numbers! $\endgroup$
    – Pavel
    Oct 18, 2014 at 17:51
  • $\begingroup$ @paulpaul1076, wolframalpha.com/input/?i=%28-1%29^{\frac26} and wolframalpha.com/input/?i=%28-1%29^{\frac13} $\endgroup$
    – lab bhattacharjee
    Oct 18, 2014 at 17:57
  • $\begingroup$ @labbhattacharjee once again, I am talking about real numbers! add "real numbers" to what you wrote and you get "indeterminate" for "2/6" exponent and "-1" for "1/3" exponent, also I edited my question $\endgroup$
    – Pavel
    Oct 18, 2014 at 18:00

2 Answers 2

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I figured it out, apparently wolfram first takes the 6th root of -1 and since (-1)^(1/6) is indeterminate since we're dealing with real numbers and can't take an even root of a negative number, it says "indeterminate".

Also, Rory's first solution is absolutely wrong (unless he corrects if when you see this), because we can't take a root of a real negative number raised to a power greater than 1, if what he wrote were true I can then prove that -1 = 1 like so: -1 = (-1)^(1/3) = (-1)^(2/6) = (1)^(1/6) = 1.

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The usual definition (used in high schools) of $x^{a/b}$ is $\sqrt[b]{x^a}$. Therefore, your expression $(-1)^{2/6}$ could mean either:

$$(-1)^{2/6}=\sqrt[6]{(-1)^2}=\sqrt[6]{1}=1$$ or $$(-1)^{2/6}=(-1)^{1/3}=\sqrt[3]{(-1)^1}=\sqrt[3]{-1}=-1$$

That seems pretty indeterminate to me!

You could complain that it is obvious that the fraction $2/6$ should be simplified before applying the definition of a rational power, but it apparently is not obvious to Wolfram Alpha. Wolfram does not know you or your particular definition, it could be argued that Wolfram took the correct approach.

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  • $\begingroup$ Then why does it not answer "1" instead of "indeterminate"? $\endgroup$
    – Pavel
    Oct 18, 2014 at 18:35
  • $\begingroup$ Because there are two obvious ways of calculating the answer, and those give different answers. If you asked me which answer were correct, I would choose $-1$. So $1$ is not at all the obvious correct answer. I made a slight edit to my answer that might help. $\endgroup$
    – Rory Daulton
    Oct 18, 2014 at 18:37
  • $\begingroup$ Yeah, I got it and posted my own answer, thanks. $\endgroup$
    – Pavel
    Oct 18, 2014 at 18:40
  • $\begingroup$ Also it's indeterminate not because it can take 2 approaches but because the answer to (-1)^(1/6) is unknown, as i said in my post $\endgroup$
    – Pavel
    Oct 18, 2014 at 18:45
  • $\begingroup$ I have actually talked about this with someone, and he said that expression x^(a/b) = b roots from x^a is only valid when x is not negative, so your first solution is incorrect $\endgroup$
    – Pavel
    Jan 9, 2015 at 1:54

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