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Sagot :

AYUDA

bjr

T se sert de a² - b² = (a+b) (a-b)

soit

29 x 31 = (30 - 1) (30 + 1) = 30² - 1²

et vous aurez donc

39 x 41 = (40 - 1) (40 + 1) = 40² - 1² = 1600 - 1 = 1599

etc :)

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