Re: Only real solutions for a=a^0.7*b

• To: mathgroup at smc.vnet.net
• Subject: [mg119729] Re: Only real solutions for a=a^0.7*b
• From: Bob Hanlon <hanlonr at cox.net>
• Date: Sun, 19 Jun 2011 07:24:59 -0400 (EDT)

```eq = a == a^0.7*b;

DeleteCases[
Select[Solve[eq, a], FreeQ[#, Complex] &],
_?((a /. #) == 0 &)]

{{a -> 1.*b^(10/3)}}

DeleteCases[
Select[Solve[Rationalize[eq], a], FreeQ[#, Complex] &],
_?((a /. #) == 0 &)]

{{a -> b^(10/3)}}

Bob Hanlon

---- "niels.gierse at gmail.com" <niels.gierse at googlemail.com> wrote:

=============
Hi all!

Is there a way to make Mathematica only report the real solution for

eq = a == a^0.7*b

Solve[eq, a]

reports:
{{a -> 0.}, {a -> (-0.5 - 0.866025 I) b^(
10/3)}, {a -> (-0.5 + 0.866025 I) b^(10/3)}, {a -> 1. b^(10/3)}}

The solution I am after is the last one. Is there a way to have solve
only report that one? As the exponent is changing from case to case
selecting one like [[5]] does not help. Playing around with Refine[],
Assuming[] etc. to impose a>0 did not help.

Does someone have a hint for me how to do this? I am using Mathematica
7.0.0 (Windows, 64 bit)

Cheers,

Niels

```

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