Re: BFBug_of_Solve
- To: mathgroup at smc.vnet.net
- Subject: [mg94790] Re: [mg94780] BFBug_of_Solve
- From: Bob Hanlon <hanlonr at cox.net>
- Date: Thu, 25 Dec 2008 03:58:39 -0500 (EST)
- Reply-to: hanlonr at cox.net
Solve[x^(2/3) - x^(1/3) - 6 == 0, x]
{{x -> 27}}
x^(2/3) - x^(1/3) - 6 /. x -> -8.
-9. + 1.73205*I
The principal cube roots of (-8) are complex
(-8.)^(2/3)
-2. + 3.4641*I
(-8.)^(1/3)
1. + 1.73205*I
%% - % - 6 == %%%
True
What you are intending to Solve is
Solve[Abs[x]^(2/3) -
Sign[x]*Abs[x]^(1/3) - 6 == 0, x]
{{x -> -8}, {x -> 27}}
Bob Hanlon
On Wed, Dec 24, 2008 at 8:49 AM , Miguel wrote:
> Let the equation x^(2/3)-x^(1/3)-6=0
>
> The roots of this equation are x=27 and x=-8. But Mathematica 6.0.1
> yields:
>
> In[]: Solve[x^(2/3)-x^(1/3)-6=0,x]
> Out[]: {{x->27}}
>
> In[]: x^(2/3)-x^(1/3)-6/.x->27
> Out[]: 0
>
> In[]:= x^(2/3)-x^(1/3)-6/.x->-8.
> Out[]: -9+1.73205i
>
> Where is my error?