Re: Solve::ivar: xxx is not a valid variable. >>
- To: mathgroup at smc.vnet.net
- Subject: [mg117228] Re: Solve::ivar: xxx is not a valid variable. >>
- From: DrMajorBob <btreat1 at austin.rr.com>
- Date: Sat, 12 Mar 2011 05:08:18 -0500 (EST)
Because Solve doesn't control the "scope" of T3.
This works when the variables are undefined:
Solve[Q == m*C*(T3 - T2), T3]
{{T3 -> (Q + C m T2)/(C m)}}
But if T3 has a value:
T3 = 3777.865080388886`;
Solve[Q == m*C*(T3 - T2), T3]
Solve[Q == C m (3777.87 - T2), 3777.87]
In that case, block the variable:
Block[{T3},
T3 /. First@Solve[Q == m*C*(T3 - T2), T3]]
(Q + C m T2)/(C m)
Bobby
On Fri, 11 Mar 2011 03:34:44 -0600, Morten J=F8rgensen <mail at maarten.dk>
wrote:
> Hi guys,
> why is it that I get this message
>
> Solve::ivar: 3777.865080388886` is not a valid variable. >>
>
> while using numbers from previous calculations (Q, m, C, T2) in this
> equation
>
> Solve[Q == m * C * (T3 - T2), T3]
>
>
> Regards, Morten
>
--
DrMajorBob at yahoo.com