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Îçíà÷åííÿ 9.6.1. Íåâëàñíèì ³íòåãðàëîì ² ðîäó â³ä<br />
ôóíêö³¿ f(õ) íà ïðîì³æêó [a, +∞) (ïîçíà÷àºòüñÿ ñèìâîëîì<br />
+∞<br />
f x dx<br />
∫ ( ) ) íàçèâàºòüñÿ ãðàíèöÿ f( x) dx,<br />
a<br />
A<br />
lim<br />
A→+∞<br />
a<br />
∫ òîáòî<br />
A<br />
∫ f( x) dx = lim ∫ f( x)<br />
dx. (9.6.1)<br />
+∞<br />
a<br />
A→+∞<br />
a<br />
Ïðè öüîìó ìîæëèâ³ 3 âèïàäêè:<br />
1) ³ñíóº ñê³í÷åííà ãðàíèöÿ (9.6.1); 2) ãðàíèöÿ (9.6.1) äîð³âíþº<br />
íåñê³í÷åííîñò³; 3) ãðàíèöÿ (9.6.1) çîâñ³ì íå ³ñíóº.<br />
Ó ïåðøîìó âèïàäêó íåâëàñíèé ³íòåãðàë ² ðîäó (9.6.1)<br />
íàçèâàºòüñÿ çá³æíèì, à ó âèïàäêàõ 2) – 3) — ðîçá³æíèì.<br />
ßêùî íà ïðîì³æêó [a, +∞) f(õ) ≥ 0, òî, ç ãåîìåòðè÷íî¿<br />
òî÷êè çîðó, íåâëàñíèé ³íòåãðàë ² ðîäó âèðàæຠïëîùó íåîáìåæåíî¿<br />
îáëàñò³ (ðèñ. 9.9).<br />
äå ñ — äîâ³ëüíå ÷èñëî. Íåâëàñíèé ³íòåãðàë çë³âà ó ð³âíîñò³<br />
(9.6.2) º çá³æíèì òîä³ ³ ò³ëüêè òîä³, êîëè îáèäâà ³íòåãðàëè<br />
ó ïðàâ³é ¿¿ ÷àñòèí³ çá³æí³.<br />
Ïðèêëàä 9.6.1<br />
+∞<br />
dx<br />
A<br />
dx<br />
π<br />
∫ =<br />
2 lim ∫ =<br />
2 lim ( arctg A− arctg0)<br />
= limarctg A = .<br />
0 1+ x A→+∞ o 1+<br />
x A→+∞ A→+∞<br />
2<br />
π<br />
Îòæå ³íòåãðàë çá³æíèé, ³ éîãî çíà÷åííÿ äîð³âíþº . 2<br />
Ïðèêëàä 9.6.2<br />
dx<br />
A<br />
dx<br />
∫ = lim ∫ = lim ( ln A− ln1)<br />
= lim ln A=+∞.<br />
x x<br />
+∞<br />
1 A→+∞1<br />
A→+∞ A→+∞<br />
Ó öüîìó ïðèêëàä³ ³íòåãðàë ðîçá³æíèé.<br />
Ïðèêëàä 9.6.3<br />
+∞<br />
A<br />
( )<br />
∫ cos xdx = lim ∫ cos xdx = lim sin A − sin 0 = lim sin A .<br />
0 A→∞ 0<br />
A→+∞ A→+∞<br />
³äîìî, ùî ôóíêö³ÿ sinx íå ìຠãðàíèö³ íà íåñê³í÷åííîñò³,<br />
òîìó â öüîìó ïðèêëàä³ íåâëàñíèé ³íòåãðàë º ðîçá³æíèì.<br />
Ïðèêëàäè 9.6.4 – 9.6.5. Îá÷èñëèòè íåâëàñí³ ³íòåãðàëè:<br />
b<br />
e dx = lim e dx = lim − e = lim e − e = 1<br />
0<br />
.<br />
+∞ b<br />
− x − x 0<br />
9.6.4. (<br />
− x) (<br />
− b<br />
∫<br />
∫<br />
)<br />
0<br />
b→+∞ 0<br />
b→+∞ b→+∞<br />
Ðèñ. 9.9<br />
Àíàëîã³÷íî âèçíà÷àºòüñÿ íåâëàñíèé ³íòåãðàë ² ðîäó íà<br />
ïðîì³æêó (–∞, b].<br />
b<br />
( ) = lim ( )<br />
∫ f x dx ∫ f x dx.<br />
−∞<br />
b<br />
B→−∞B<br />
Íåâëàñíèé ³íòåãðàë ç äâîìà íåñê³í÷åííèìè ìåæàìè âèçíà÷àºòüñÿ<br />
ð³âí³ñòþ<br />
+∞ c<br />
+∞<br />
( ) = ( ) + ( )<br />
∫ f x dx ∫ f x dx ∫ f x dx. (9.6.2)<br />
−∞<br />
−∞<br />
c<br />
+∞<br />
dx<br />
0<br />
dx<br />
b<br />
dx<br />
0<br />
b<br />
9.6.5. ∫ = lim lim lim arctg lim arctg<br />
2 ∫ + 1 →−∞ 2 ∫ = + =<br />
1 →+∞ 2<br />
−∞ x + a<br />
a x + b<br />
0 x + 1 a→−∞ a b→+∞<br />
0<br />
⎛ π⎞<br />
π<br />
=−arctg( −∞ ) + arctg( +∞ ) =−⎜− ⎟+ =π.<br />
⎝ 2⎠<br />
2<br />
Ïðèêëàä 9.6.6 (òåîðåòè÷íèé). Âèçíà÷èòè, ïðè ÿêèõ çíà-<br />
÷åííÿõ λ º çá³æíèì íåâëàñíèé ³íòåãðàë:<br />
+∞<br />
dx<br />
I( λ ) = ∫ λ . (9.6.3)<br />
1 x<br />
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