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

  • To: mathgroup at smc.vnet.net
  • Subject: [mg119726] Re: Only real solutions for a=a^0.7*b
  • From: David Bailey <dave at removedbailey.co.uk>
  • Date: Sun, 19 Jun 2011 07:24:26 -0400 (EDT)
  • References: <itjdqu$kvd$1@smc.vnet.net>

On 19/06/2011 00:54, niels.gierse at gmail.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
>
>
Probably the easiest answer, is to evaluate
Solve[a==a^x*b,a]

You can pick the answer you require and simply use the formula for 
different values of x.

In general, I would try not to solve equations containing real numbers.

David Bailey
http://www.dbaileyconsultancy.co.uk


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