2.1. Rekenkundige rij
Verschil (v) : bij iedere stap n wordt het verschil v er bij opgeteld
• directe formule : an = a1 + (n – 1) v
• recursieve formule : an = an-1 + v met a1 = begingetal
{an }∞
n=1 = {5, 9, 13, ...} ⇒ v = 4 en a1 = 5
directe formule an = 5 + 4(n – 1) = 1 + 4n
recursieve formule an = an‒1 + 4 en a1 = 5
• verschilrij is de rij {a2 – a1, a3 – a2, ..... , an – an‒1}
• de som van de eerste n termen is ∑n1 sn = ½ n (a1 + an)
2.2. Meetkundige rij
Reden (r) : bij iedere stap n wordt met de reden r vermenigvuldigd
• directe formule : an = a1 · r(n ‒ 1)
• recursieve formule : an = an-1 · r met a1 = begingetal
{an }∞
n=1 = {2, 6, 18, ...} ⇒ r = 3 en a1 = 2
Directe formule an = 2 · 3(n – 1)
Recursieve formule an = 3 · an‒1 en a1 = 2
2.3. Stellingen
• als lim an bestaat is de rij convergent, anders divergent
n→∞
• als lim |an | = 0 dan lim an = 0. Alleen gebruiken als de limiet naar 0 gaat
n→∞ n→∞
• insluitstelling: als voor alle n geldt an ≥ bn ≥ cn en lim an = lim cn dan lim bn = lim an
n→∞ n→∞ n→∞ n→∞
• r<0 : lim nr = 0
n→∞
• r=0 : lim nr = 1
n→∞
• r>0 : lim nr is divergent
n→∞
• –1 < r < 1 : lim r n = 0
n→∞
• r=1 : lim r n = 1
n→∞
• r ≤ ‒1 ꓦ r > 1 : lim r n is divergent
n→∞
• voor convergente rijen geldt lim an = lim an+1
n→∞ n→∞
2.4. Begrensdheid
Een rij is convergent wanneer het voldoet aan:
• stijgende rij : Ɐ n ϵ ℕ+ , ∃ M ϵ ℝ | a(n+1) > an ꓥ M ≥ an
• dalende rij : Ɐ n ϵ ℕ+ , ∃ m ϵ ℝ | a(n+1) < an ꓥ m ≤ an
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