Alternativ, se poate spune si reductio ad absurdum. Desi in romana fraza e folosita aproape exclusiv in domenii "tehnice" (matematica, logica, filosofie), aici se foloseste in mod curent ca atare, in politica sau in viata de zi cu zi. Dar daca nu merge in context, se poate si cum am spus mai sus.

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Vezi o explicaţie aici:http://en.wikipedia.org/wiki/Reductio_ad_absurdum
Nu este un exerciţiu, aşa cum zici, dar expresia se foloseşte la rezolvarea problemelor de matematică, eu aşa înţeleg.
Pentru expresia dacă dai o căutare pe google, cred că te edifici.

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Note added at 4 hrs (2007-11-23 17:12:13 GMT)
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Am găsit şi ”assume by absurd”, un simplu sinonim, dealtfel...

fara un context mai larg, este o pura speculatie.

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Note added at 1 day5 hrs (2007-11-24 18:16:06 GMT)
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Parerea mea este ca e o idioma romaneasca si as adapta-o un pic de genul " if taken (whatever this is) to an extreme/if we take it to an extreme...". In engleza, se spune frecvent"let's say/let's assume"fara nota asta extrema/absurda.

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Note added at 1 day13 hrs (2007-11-25 02:17:26 GMT)
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Ca sa fiu sincera , nici "let's assume against all reason" nu l-am auzit ca atare desi e o traducere fidela dar nu neaparat "idiomatica'. Mai degraba, "this defies reason"(absurd) si nu ar reda contextul oricum. Contextul nefiind matematic/stiintific, nu se preteaza la expresii latine (Reductio..)

Le raisonnement par l'absurde in franceza
in engleza e ca-n latina
Reductio ad absurdum (Latin: "reduction to the absurd") also known as an apagogical argument, reductio ad impossibile, or proof by contradiction, is a type of logical argument where one assumes a claim for the sake of argument, derives an absurd or ridiculous outcome, and then concludes that the original assumption must have been wrong as it led to an absurd result.

It makes use of the law of non-contradiction — a statement cannot be both true and false. In some cases it may also make use of the law of excluded middle — a statement must be either true or false. The phrase is traceable back to the Greek ἡ εἰς ἄτοπον ἀπαγωγή (hē eis átopon apagōgḗ), meaning "reduction to the absurd", often used by Aristotle.
http://en.wikipedia.org/wiki/Reductio_ad_absurdum

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Note added at 3 days11 hrs (2007-11-26 23:58:20 GMT)
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iar intr-o fraza using the "reductio ad absurdum " method

sau

"using an apagogical argument"