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A4-format til udskrift. - Aarhus Universitet

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Det uegentlige integral<br />

er divergent.<br />

Det uegentlige integral<br />

er konvergent.<br />

1. L’HOSPITALS REGEL OG UEGENTLIGE INTEGRALER 115<br />

∞<br />

1<br />

t<br />

1<br />

dx = lim<br />

x t→∞<br />

1<br />

∞<br />

1<br />

= lim<br />

t→∞ lnt = ∞<br />

t<br />

1<br />

dx = lim<br />

x2 t→∞<br />

1<br />

1<br />

dx = lim<br />

x t→∞ [lnx]t 1<br />

1<br />

dx = 1<br />

x2 1.18. Uendelige intervaller ☞ [S] 5.10 Improper integrals<br />

y<br />

1 t<br />

Uendeligt interval, uendeligt areal<br />

y = 1<br />

x<br />

1.19. Arctan integral ☞ [S] 5.10 Improper integrals<br />

Eksempel 3<br />

∞<br />

1<br />

dx = π<br />

−∞ 1 + x2 Løsning<br />

t<br />

−∞<br />

0<br />

1<br />

dx = [Arctanx]t<br />

1 + x2 0 = Arctan t<br />

Grænseovergange limt→±∞ Arctan t = ± π<br />

2 giver<br />

∞ 0 ∞<br />

1<br />

1<br />

1<br />

dx = dx + dx = π<br />

1 + x2 −∞ 1 + x2 0 1 + x2 1.20. Reciprok potens ☞ [S] 5.10 Improper integrals<br />

Eksempel 4<br />

Det uegentlige integral p = 1<br />

∞<br />

1<br />

1<br />

dx = lim<br />

xp t→∞<br />

= lim<br />

t→∞<br />

t<br />

er konvergent for p > 1 med værdi ∞<br />

1<br />

1<br />

1<br />

1 − p<br />

<br />

1<br />

1 − p ·<br />

<br />

− 1<br />

tp−1 1<br />

dx = lim<br />

xp t→∞<br />

<br />

1<br />

1 1<br />

dx =<br />

xp p − 1<br />

x<br />

1<br />

xp−1 t 1

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