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Matamaitic » Loighic & Tacartheoiric Úsáidtear an modh seo chun ráitis san fhoirm: do gach x sa tacar X, tá an abairt oscailte p(x) fíor a chruthú. Chun a leithéid de dhearbhú a chruthú, cuirimid chun frithshampla a aimsiú. Is é sin le rá, lorgaímid luach amháin de chuid x dá bhfuil an ráiteas bréagach; agus ó bhí an ráiteas in ainm is a bheith fíor do gach luach de chuid x, cruthaíonn an frithshampla amháin seo an ráiteas a bheith bréagach.
Mathematics » Logic & Set Theory This method is used to disprove statements of the form: for all x in set X, the open sentence p(x) is true. To disprove such an assertion, we proceed to find a counter-example. In other words we look for one value of x for which the statement is false; and since the statement was supposed to be true for all values of x, this single counter example disproves the statement.