Re: Solving Equations with 3 variables and many parameters

• To: mathgroup at smc.vnet.net
• Subject: [mg132379] Re: Solving Equations with 3 variables and many parameters
• From: Bob Hanlon <hanlonr357 at gmail.com>
• Date: Sun, 2 Mar 2014 21:26:14 -0500 (EST)
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• References: <20140302060452.5EAEC69E6@smc.vnet.net>

```You need to use Equal ( == ) in the equations rather than Set ( = ).

eqns1 = {P1 == (a[1] + C*b[11] + b[12]*P2)/(2*b[11]),
P2 == (a[2] + C*b[22] + b[21]*P1)/(2*b[22])};

sol1 = Solve[eqns1, {P1, P2}];

eqns1 /. sol1 // Simplify

{{True, True}}

eqns2 = {P1 == (a[1] + C*b[11] + b[12]*P2 + b[13]*P3)/(2*b[11]),
P2 == (a[2] + C*b[22] + b[21]*P1 + b[23]*P3)/(2*b[22]),
P3 == (a[3] + C*b[33] + b[31]*P1 + b[32]*P2)/(2*b[33])};

sol2 = Solve[eqns2, {P1, P2, P3}];

eqns2 /. sol2 // Simplify

{{True, True, True}}

Bob Hanlon

On Sun, Mar 2, 2014 at 1:04 AM, drascher <rascher at usfca.edu> wrote:

> Hello,
>
> I am able to get Mathematica to solve:
>
> Solve[{P1=(a[1]+C*b[11]+b[12]*P2)/(2*b[11]),
> P2=(a[2]+C*b[22]+b[21]*P1)/(2*b[22])}, {P1, P2}]
>
> but it won't solve the following (which is one more variable P3 and its
> equation):
>
> Solve[{P1=(a[1]+C*b[11]+b[12]*P2+b[13]*P3)/(2*b[11]),
> P2=(a[2]+C*b[22]+b[21]*P1+b[23]*P3)/(2*b[22]),
> P3=(a[3]+C*b[33]+b[31]*P1+b[32]*P2)/(2*b[33])}, {P1, P2, P3}]
>
> It says that it doesn't understand how to interpret the word "Solve" even
> though it does it for the first eqn.
>
> Any help is appreciated.
>
>

```

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