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rconan
Newbie
Joined: 19 Sep 2005
Posts: 3
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 Posted: Mon Sep 19, 2005 1:16 pm Post subject: Cube root of a negative number |
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Why can Openoffice.org Calc not work out the cube root of a negative power?
Is it just the syntax I am using or is it actually a missing feature?
My syntax:
=(-8)^(1/3)
should give -2 but instead shows #VALUE |
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richhill
OOo Advocate
Joined: 16 Jun 2004
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Location: Mesa, AZ
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rconan
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Joined: 19 Sep 2005
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| Posted: Tue Sep 20, 2005 5:16 am Post subject: |
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Does -8 have three cube roots?
I can only find one: -2
Does Xnumbers support cube rooting negative numbers? |
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9point9
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Joined: 31 Aug 2004
Posts: 3875
Location: UK
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| Posted: Tue Sep 20, 2005 5:49 am Post subject: |
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There is also 2i^(2/3). The other one I can't think of for the moment.
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Ed
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Joined: 28 May 2003
Posts: 1040
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| Posted: Tue Sep 20, 2005 5:54 am Post subject: |
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| rconan wrote: | Does -8 have three cube roots?
I can only find one: -2
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-2 is the only real cube root of -8, but 1 + √3 i and 1 - √3 i are also cube roots of -8. |
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rconan
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Joined: 19 Sep 2005
Posts: 3
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| Posted: Tue Sep 20, 2005 7:13 am Post subject: |
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| Is there a way to find the REAL cube root of a negative number in Oo.o calc or do I have to convert my entire coursework to excel format and do it in windoze? |
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Ed
Super User
Joined: 28 May 2003
Posts: 1040
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| Posted: Wed Sep 21, 2005 10:59 am Post subject: |
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| rconan wrote: | | Is there a way to find the REAL cube root of a negative number in Oo.o calc or do I have to convert my entire coursework to excel format and do it in windoze? |
If the cube root always works for a positive number, you could try | Code: | | =sign(A1)*(abs(A1)^(1/3)) |
to get the real cube root of A1 whether is's positive or negative.
Why it would work for a positive number and not a negative one is a mystery. All numbers have three cube roots, so why it can give the real value for a positive number but not a negative one I don't know. |
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Dale
Super User
Joined: 21 Feb 2005
Posts: 1440
Location: Australia
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| Posted: Wed Sep 21, 2005 4:25 pm Post subject: |
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| Ed wrote: | | Why it would work for a positive number and not a negative one is a mystery. |
Just a guess: Calc might use logarithms to compute (fractional) powers.
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To err is human, but to destroy your slippers in the process takes a real son of a bitch: Me!
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David
Super User
Joined: 24 Oct 2003
Posts: 5668
Location: Canada
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| Posted: Wed Sep 21, 2005 5:18 pm Post subject: |
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| rconan wrote: | | Is there a way to find the REAL cube root of a negative number in Oo.o calc or do I have to convert my entire coursework to excel format and do it in windoze? |
The real cube root of a negative number is always negative. Find the real positive root and append the - sign.
David. |
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