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Submitted on: 28 May 2002.
Revised on: 25 September 2002.
Accepted on: 21 November 2002.
___________________Avances de Investigacion en Educacion Matematica
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Submitted on: 13 May 2009.
Revised on: 21 June 2009.
Accepted on: 09 July 2009.
___________________Avances de Investigacion en Educacion Matematica