Bonjour
uₙ₊₁ = 1/(2(n + 1) + 1) = 1/(2n + 3)
uₙ₊₁ - uₙ = 1/(2n + 3) - 1/(2n + 1) = (2n + 1)/[(2n+1)(2n+3)] - (2n + 3)[(2n + 1)(2n +3)]
uₙ₊₁ - uₙ = (2n + 1 - 2n - 3)/[(2n + 1)(2n + 3)]
uₙ₊₁ - uₙ = -2/[(2n + 1)(2n + 3)]
comme n est un entier naturel, (2n + 1)(2n + 3) > 0
donc -2/[(2n + 1)(2n + 3)] < 0
donc uₙ₊₁ - uₙ < 0
et donc uₙ₊₁ < uₙ
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Bonjour
uₙ₊₁ = 1/(2(n + 1) + 1) = 1/(2n + 3)
uₙ₊₁ - uₙ = 1/(2n + 3) - 1/(2n + 1) = (2n + 1)/[(2n+1)(2n+3)] - (2n + 3)[(2n + 1)(2n +3)]
uₙ₊₁ - uₙ = (2n + 1 - 2n - 3)/[(2n + 1)(2n + 3)]
uₙ₊₁ - uₙ = -2/[(2n + 1)(2n + 3)]
comme n est un entier naturel, (2n + 1)(2n + 3) > 0
donc -2/[(2n + 1)(2n + 3)] < 0
donc uₙ₊₁ - uₙ < 0
et donc uₙ₊₁ < uₙ