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MATEMAATILINE ANALÜÜS II - Tallinna Tehnikaülikool

MATEMAATILINE ANALÜÜS II - Tallinna Tehnikaülikool

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3.4. KAHEKORDSE INTEGRAALI RAKENDUSED 161<br />

Rakendame m~olema osa jaoks valemit (3.3.4) eraldi. Saame<br />

S D = ∫∫<br />

dx dy =<br />

D<br />

∫<br />

arctan 2<br />

0<br />

dϕ<br />

2 sin ∫ ϕ<br />

0<br />

ρ dρ +<br />

∫π/2<br />

arctan 2<br />

dϕ<br />

4 cos ∫ ϕ<br />

0<br />

ρ dρ =<br />

=<br />

∫<br />

arctan 2<br />

0<br />

dϕ ρ2<br />

2<br />

∣<br />

2 sin ϕ<br />

0<br />

+<br />

∫π/2<br />

arctan 2<br />

dϕ ρ2<br />

2<br />

∣<br />

4 cos ϕ<br />

0<br />

=<br />

=<br />

∫<br />

arctan 2<br />

0<br />

2 sin 2 ϕdϕ +<br />

∫π/2<br />

arctan 2<br />

8 cos 2 ϕdϕ =<br />

=<br />

=<br />

∫<br />

arctan 2<br />

0<br />

(<br />

ϕ −<br />

(1 − cos 2ϕ)dϕ +<br />

∫π/2<br />

arctan 2<br />

4(1 + cos 2ϕ)dϕ =<br />

)∣<br />

sin 2ϕ ∣∣∣<br />

arctan 2<br />

+ (4ϕ + 2 sin 2ϕ)| π/2<br />

arctan 2<br />

2<br />

=<br />

0<br />

= 2π − 3 arctan 2 − 2. ♦<br />

3.4 Kahekordse integraali rakendused<br />

3.4.1 Tasandilise pinnatüki pindala arvutamine<br />

Kui D on kinnine t~okestatud ühelisidus hulk xy-tasandil, siis Lause 3.1.2<br />

p~ohjal<br />

∫∫<br />

S D =<br />

D<br />

dxdy. (3.4.1)<br />

Näide 1. Leiame joontega y = x 2 ja y = x + 2 määratud piirkonna D<br />

pindala.<br />

Skitseerime piirkonna D

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