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Re: simple equation substitutions

  • To: mathgroup at smc.vnet.net
  • Subject: [mg128547] Re: simple equation substitutions
  • From: Bob Hanlon <hanlonr357 at gmail.com>
  • Date: Fri, 2 Nov 2012 00:45:26 -0400 (EDT)
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  • References: <20121101071928.99D326857@smc.vnet.net>

eqns = {y == x + 2, x == 7};

Solve[eqns, {x, y}]

{{x -> 7, y -> 9}}

Solve[eqns, Variables[eqns]]

{{x -> 7, y -> 9}}

Simplify[Solve[eqns[[1]], y], eqns[[2]]]

{{y -> 9}}

Simplify[Solve[#[[1]], y], #[[2]]] &@eqns

{{y -> 9}}

Solve[eqns, y, Integers]

{{y -> ConditionalExpression[9, x == 7]}}

Reduce[eqns, {x, y}] // ToRules

{x -> 7, y -> 9}

Reduce[eqns, y] // ToRules

{x -> 7, y -> 9}

Reduce[eqns, x] // ToRules

{y -> 9, x -> 7}

Eliminate[eqns, x] // ToRules

{y -> 9}


Bob Hanlon


On Thu, Nov 1, 2012 at 3:19 AM, Neal Becker <ndbecker2 at gmail.com> wrote:
> {y == x + 2, x == 7}
>
> Solve[%, y]
>
> -> {}
>
> What I expected to happen is have x->7 applied to y == x+2.
>
> My real plan is to develop a list of (a lot more complicated) expressions that will have repeated substitutions applied.
>
> Why didn't the above work and/or what should I do instead?
>



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