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