Cubic root (8) = 2 ?  Printable Version + HP Forums (https://archived.hpcalc.org/museumforum) + Forum: HP Museum Forums (https://archived.hpcalc.org/museumforum/forum1.html) + Forum: Old HP Forum Archives (https://archived.hpcalc.org/museumforum/forum2.html) + Thread: Cubic root (8) = 2 ? (/thread247794.html) 
Cubic root (8) = 2 ?  Gilles Carpentier  08042013 What do you think of this :
All seems perfect and nothing seems violate math rules... but....
Edited: 4 Aug 2013, 6:22 p.m.
Re: Cubic root (8) = 2 ?  Csaba Tizedes (Hungary)  08042013 At the first step the equality is not true, the "problem" is similar like SQRT(x^2) is not equal with x.
When you are expanded the 1/3 to 2/6, you are make SQRT(x^2)==x mistake.
Re: Cubic root (8) = 2 ?  CompSystems  08042013 [HPPrime CAS] approx(evalc((8)^(1/3))) [Enter] return 1+73...i OK (principal complex root) but exact(1+73...i) [Enter] .... /=/ 2 (1)^(1/3) =( approx((8)^(1/3)) [Enter] return 2 real root Request for HPPrime rename evalc to evalCplx Thanks more info
http://www.wolframalpha.com/input/?i=%288%29%5E%281%2F3%29
Re: Cubic root (8) = 2 ?  Harald  08052013 The first step is perfectly fine. It is the third step that is the problem. Edited: 5 Aug 2013, 4:53 a.m.
Re: Cubic root (8) = 2 ?  Walter B  08052013 Watch it: 64^{1/6} = ±2 !
Re: Cubic root (8) = 2 ?  Reth  08052013 Quote:BS Re: Cubic root (8) = 2 ?  Gilles Carpentier  08052013 So we can say that
sqrt(4)=±2
Re: Cubic root (8) = 2 ?  Walter B  08052013 I hate opening that box once again, but there's a "convention" declaring SQRT(4) = +2 only. OTOH, 4^{1/2} = ± 2 since it inverts (±2)^{2} = 4.
Re: Cubic root (8) = 2 ?  Csaba Tizedes (Hungary)  08052013 ...well, you're wrong, but my hungarian thinking gives me an other point of view when i'll try to solve similar problems... For example:
(1)^1 = 1, but The mathematical symbol (...)^c == (...)^(a/b), where a/b = ccontains the mistake not the mathematical operation (which have not even been performed).
You do not have to make error to know that a mistake.
Csaba
Re: Cubic root (8) = 2 ?  fhub  08052013 Quote:So you want to say that 1/3 = 2/6 is wrong?? LOL, what madhouse is this here?
Franz
Re: Cubic root (8) = 2 ?  Thomas Klemm  08052013 Exponentiation is rightassociative, thus:
Cheers Re: Cubic root (8) = 2 ?  Ángel Martin  08062013 So much ado just to come to terms with the fact that square root is a multivalue function... that's all there is to it, nothing mysterious or bizarre. Sure enough not a matter of "conventions" or "dual rules" or "exceptions", but plain and simple math.
Cheers, Re: Cubic root (8) = 2 ?  Victor Koechli  08062013 I still don't get it. The cubic root has three solutions, two complex and one real. What transformation in Gilles' process intoduces +2 as a fourth solution? Expanding 1/3 to 2/6? Re: Cubic root (8) = 2 ?  Csaba Tizedes (Hungary)  08062013 :D Of course, 1/3 = 2/6is true, BUT (8)^(1/3) = (8)^(2/6)is NOT true.
(8)^(1/3) == ((8)^(1)) ^ (1/3) == (8) ^ (1/3) = 2
My english is not very good, so one another example for clarification: A little BASIC program: Csaba
Edited: 6 Aug 2013, 3:20 a.m.
Re: Cubic root (8) = 2 ?  fhub  08062013 Quote:That's pure nonsense! The error is NOT in replacing 1/3 by 2/6, but in what you do with this 2/6 later. Of course it's wrong to put this numerator 2 to the base (8) first and then do the exponentiation ^(1/6), i.e. (8)^(2/6) is NOT the same as ((8)^2)^(1/6), simply because the rule a^(b*c)=(a^b)^c is NOT valid for ALL real numbers! So the 'error' in the starting post is indeed in the 3rd expression (or in the 2nd step)  besides that there's still another error in having forgotten a pair of important parentheses (because exponentiation is rightassociative as already mentioned by an other member). Franz
Edited: 6 Aug 2013, 4:05 a.m.
Re: Cubic root (8) = 2 ?  Gilles Carpentier  08062013 ...
Edited: 6 Aug 2013, 4:18 a.m.
Re: Cubic root (8) = 2 ?  Eddie W. Shore  08062013 The sixth roots of 64 are: 2, 2, square_root(3)*i+1,square_root(3)*i+1, square_root(3)*i1, square_root(3)*i1
Re: Cubic root (8) = 2 ?  Eddie W. Shore  08062013 Also, with the NTHROOT functions, the calculator strives to give the principal root. If you want all the roots and have a calculator with CAS abilities, we will need to use a cSolve function, like this: CSolve(x^38=0,x)
Re: Cubic root (8) = 2 ?  Gilles Carpentier  08062013 Here are all (?) the ways to do this with the 50G
You can notice than even in Real Approx mode, the 50G returns a complex number (it's a legacy of the 48 series where there was no 'Complex mode') and because (8)^0.33333333 is a complex number (no real root).It should result an error (no solution in R) However, returning (2) in 'C~' mode with XROOT is not logic (It must retunr the principal root); in 'R=' mode, XROOT and ^ should return (2) and not a warning to switch in C
Edited: 6 Aug 2013, 7:08 p.m.
Re: Cubic root (8) = 2 ?  Kiyoshi Akima  08062013 What about [ 1 0 0 8 ] PROOT to get the roots of the polynomial x^3 + 8 = 0 ?
Re: Cubic root (8) = 2 ?  Chris Osburn  08062013 Quote:[(1.000,1.732) (1.000,1.732) (2.000,0,000)] Re: Cubic root (8) = 2 ?  Gilles Carpentier  08072013 ;)
and it's <> 64^(1/6)
Re: Cubic root (8) = 2 ?  Eddie W. Shore  08092013 (8)^(1/3) is taking finding the three roots of 8
(8)^(2/6) is taking the the six roots of (8)^2, or 64
Re: Cubic root (8) = 2 ?  fhub  08092013 Quote:Mathematics: fail Re: Cubic root (8) = 2 ?  Gilles Carpentier  08122013 Both with 50G and Prime CAS : a^b^c is not (a^b)^c but a^(b^c)
Exponentiation is a curious beast : rightassociative as wrote Thomas.
Re: Cubic root (8) = 2 ?  fhub  08122013 Quote:Yes, of course I know this, but what has this to do with your (wrong!) assumption that (8)^(2/6) has to be (or should be) calculated as ((8)^2)^(1/6) ???
Your previous statement "(8)^(2/6) is taking the the six roots of (8)^2, or 64" would require the following transformation: Franz
Edited: 12 Aug 2013, 4:25 a.m.
Re: Cubic root (8) = 2 ?  Gilles Carpentier  08122013
Quote: I think we said the same things ... The idea of my post was (like a puzzle) to show that ^ is right associative (as far I know it's the only operator like this ?) (8)^(2/6)= (8)^2^(1/6) is true but (8)^(2/6)=((8)^2)^(1/6) is false And of course (8)^2^1/6 <> 64^(1/6). Here is the 'error' in the initial post
Edited: 12 Aug 2013, 6:02 a.m.
Re: Cubic root (8) = 2 ?  fhub  08122013 Quote:No, it's not! The RHS would mean to calculate the 6th root of 2 first, and that's something completely different!
Franz
Re: Cubic root (8) = 2 ?  Gilles Carpentier  08122013 Oh ! I see it now !
Thanks
Re: Cubic root (8) = 2 ?  jep2276  08122013 Quote: Yes, when the base is negative. The rational exponent method cannot be used with negative bases because it does not have continuity.
Quote:yes, see above! Then again, see above, if the base is negative then, the rational exponent method cannot be used so 1/3 should not be changed to 2/6! Edited: 12 Aug 2013, 11:55 a.m.
Re: Cubic root (8) = 2 ?  jep2276  08122013 Just google Rational Exponent Method with Negative Base and it will be found that it does not apply (that way it is not me stating why). So 1/3 cannot be changed to 2/6 with the Rational Exponent Method.
Re: Cubic root (8) = 2 ?  fhub  08122013 Quote:Correct! Quote:Well, it should not be changed (simply because it doesn't make any sense at all), but it definitely can be changed (if you want), because 1/3=2/6=5/15=.....=70/210=..... etc., no matter in which expression or formula this subexpression is contained. So as I've stated already a few times before: replacing 1/3 by 2/6 is NOT any error (it's just useless), the error is how the original calculation proceeded with this 2/6! And even if the base would have been positive (i.e. 8 instead of 8), an expression 8^(2/6) has to be calculated as 8^(1/3) but NOT as (8^2)^(1/6). Although it's not really wrong in this case, but the expression (2/6) has definitely priority over 8^... (ever heard of PEMDAS?), because 2/6 CAN be simplified to 1/3.
Franz
Re: Cubic root (8) = 2 ?  jep2276  08122013 You mean we should use fractions that have been reduced! Ok, I can buy that. Still, the Rational Exponent Method does not apply to a negative base so the entire argument is axiomatic. And, Quote: Re: Cubic root (8) = 2 ?  fhub  08122013 Quote:I'm glad you finally got it  although it took quite some time! ;) So peace again from now on ... :) Franz
Edited: 12 Aug 2013, 12:55 p.m.
Re: Cubic root (8) = 2 ?  jep2276  08122013 I have always got it. Disagreement does not translate to ignorance; however, acceptance of differences leads to greatness. Putting aside the argument of application, I still disagree with the entire concept of teaching math with a nursery rhyme, i.e. PEMDAS, because I feel that leads to only teaching the manipulation of numbers and not the understanding of math. Similarly, I think this thread is somewhat a product of that same type of problem. Re: Cubic root (8) = 2 ?  George Litauszky  08122013 Quote:I think: If the base is negative and the exponent is even, the result is positive, and if the exponent is odd, the result is negative.
So, this expression is true only:
Because SQRT(X^2)= +X as Csaba Tizedes wrote.
Edited: 12 Aug 2013, 3:55 p.m.
Re: Cubic root (8) = 2 ?  Thomas Klemm  08122013 Quote: Otherwise a^{bc} = a^{bc}, thus we wouldn't gain much by this convention.
Quote:
Any assignment operators are also typically rightassociative.
Re: Cubic root (8) = 2 ?  jep2276  08122013 The rational exponent method cannot be used with negative bases because it does not have continuity Quote:so it is not continuous! f(q)=b^{q} is continuous for b>0 but it is not continuous for b<0 and more importantly, when b<0 it is not continuous for the rational set of q for which it is defined. Anyway, I thought I was not going to explain this ....
John Edited: 12 Aug 2013, 10:34 p.m.
