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What is the domain of f(x)=x^(2/3)?

  • Thread starter MSchott
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What is the domain of f(x)=x^(2/3)?

If I take a negative number and square it, the result is positive. For example, (-4)^2 is 16. The cube root is 16 is also a positive number. So I conclude that the domain of this function is "all real numbers". But in Wolfram the answer is "all non-negative real numbers". Where is my thinking going awry?
 

Answers and Replies

  • #2
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I don't think that [itex]a^{b/c} = \sqrt[c]{a^b}[/itex] can be used when a is negative.
 
  • #3
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acabus: I am still having difficulty understanding. If "b" is an even power doesn't the negative "a" under the square root sign become positive?
 
  • #4
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What is the domain of f(x)=x^(2/3)?

If I take a negative number and square it, the result is positive. For example, (-4)^2 is 16. The cube root is 16 is also a positive number. So I conclude that the domain of this function is "all real numbers". But in Wolfram the answer is "all non-negative real numbers". Where is my thinking going awry?
You are correct - the domain of your function is all real numbers. The reason, I believe, that WolframAlpha gives a different domain is that it is converting x2/3 to a different form, using e as the base rather than x.

y = x2/3
=> ln(y) = (2/3) ln(x)

=> eln(y) = e(2/3)ln(x)
=> y = e(2/3)ln(x)

Now, x must be greater than 0. This is almost the same as what WA shows, except for zero.
 
  • #5
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Mark 44 Thank you so much.
 

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