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Re: How to isolate the constant in a two variable

  • To: mathgroup at smc.vnet.net
  • Subject: [mg113482] Re: How to isolate the constant in a two variable
  • From: Bob Hanlon <hanlonr at cox.net>
  • Date: Sat, 30 Oct 2010 04:39:06 -0400 (EDT)

expr = 4 X*X + 3 Y*Y + 2 X*Y + 1;

If there is always a constant

expr[[1]]

1

But note that if there is no constant then

(expr - 1)[[1]]

4 X^2

Fix with

Module[{c = expr[[1]]},
 If[NumericQ[c], c, 0]]

1

The following are more robust

Select[expr, NumericQ]

1

Using Coefficient

Fold[Coefficient[#1, #2, 0] &,
 expr, Variables[expr]]

1

Using coefficientList

Module[{vars = Variables[expr]},
 CoefficientList[expr,
   vars][[Sequence @@ Table[1, {Length[vars]}]]]]

1


Bob Hanlon

---- "Eduardo M. A. M.Mendes" <emammendes at gmail.com> wrote: 

=============
Hello

 

I know that Coefficient can isolate the coefficient relate to the variables
in an equation but I wonder how the constant could be retrieved.

 

Something like

 

  4X*X+3Y*Y+2X*Y+1

 

How to get to isolate the constant 1?

 

Many thanks

 

Ed 




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