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ÂÏÐÀÂÈ<br />
Âèçíà÷èòè ³íòåðâàë çá³æíîñò³ ñòåïåíåâèõ ðÿä³â:<br />
12.27.<br />
12.29.<br />
n<br />
∞ ( −x<br />
∞<br />
n<br />
)<br />
∑ ; 12.28. 2 !<br />
1<br />
1 3<br />
n−<br />
( )<br />
n=<br />
n=<br />
1 2 n !<br />
x<br />
n x<br />
2n<br />
∑ ;<br />
∞ ( x + 8) 3n<br />
∞<br />
∑ ; 12.30. 2 n<br />
10 ( 2x<br />
3) 2n<br />
−<br />
−<br />
1<br />
n=<br />
1<br />
∞<br />
n<br />
2<br />
∑ ;<br />
n=<br />
1<br />
2<br />
12.31. ( 2) n n<br />
∑ − x ; 12.32. ∑ ( −1)<br />
n=<br />
0<br />
∞<br />
n=<br />
1<br />
n−1<br />
12.5.4. Âëàñòèâîñò³ ñòåïåíåâèõ ðÿä³â<br />
Íåõàé ôóíêö³ÿ f(x) º ñóìîþ ñòåïåíåâîãî ðÿäó<br />
( )<br />
2<br />
n<br />
=<br />
0<br />
+<br />
1<br />
+<br />
2<br />
+ +<br />
n<br />
+<br />
( ) 2 n−<br />
x − 4<br />
1<br />
2n<br />
−1<br />
f x a a x a x K a x K, (12.5.7)<br />
³íòåðâàë çá³æíîñò³ ÿêîãî (–R, R).<br />
Ó öüîìó âèïàäêó êàæóòü, ùî íà ³íòåðâàë³ (–R, R) ôóíêö³ÿ<br />
f(x) ðîçêëàäàºòüñÿ â ñòåïåíåâèé ðÿä (àáî â ðÿä çà ñòåïåíÿìè<br />
x) ³ ìàþòü ì³ñöå òàê³ äâ³ âëàñòèâîñò³. Íàâåäåìî ¿õ<br />
áåç äîâåäåííÿ:<br />
1) ÿêùî ôóíêö³ÿ f(x) íà ³íòåðâàë³ (–R, R) ðîçêëàäàºòüñÿ<br />
â ñòåïåíåâèé ðÿä (12.5.7), òî ìîæóòü áóòè âèçíà÷åí³ ¿¿ ïîõ³äí³<br />
áóäü-ÿêîãî ïîðÿäêó íà öüîìó æ ³íòåðâàë³. Ïðè öüîìó<br />
â³äïîâ³äí³ ðÿäè ìàþòü òîé ñàìèé ³íòåðâàë çá³æíîñò³, ùî ³<br />
ðÿä (12.5.7) Íàïðèêëàä,<br />
′<br />
2<br />
n<br />
f′ x = a + a x+ a x + K+ a x + K =<br />
( ) ( 0 1 2<br />
n )<br />
= a + 2a x+ 3a x + K+ na x − + K;<br />
2 n 1<br />
1 2 3<br />
n<br />
2) ÿêùî ôóíêö³ÿ f(x) íà ³íòåðâàë³ (–R, R) ðîçêëàäàºòüñÿ<br />
â ñòåïåíåâèé ðÿä (12.5.7), òî âîíà ³íòåãðîâíà â ³íòåðâàë³<br />
(–R, R) é ³íòåãðàë â³ä íå¿ ìîæå áóòè îá÷èñëåíèé ïî÷ëåííèì<br />
³íòåãðóâàííÿì ðÿäó (12.5.7), òîáòî ÿêùî x 1 , x 2 ∈(–R, R),<br />
òî<br />
.<br />
x2 2<br />
x<br />
2<br />
n<br />
( ) ( 0 1 2<br />
n )<br />
∫ f x dx= ∫ a + ax+ ax + K+ a x + K dx=<br />
x1 x1<br />
x2 x2 x2 x2<br />
2<br />
n<br />
∫ a0dx ∫ a1xdx ∫ a2x dx ∫ an<br />
x dx<br />
x1 x1 x1 x1<br />
+ + + K+ + K.<br />
Ñòàíîâèòü ³íòåðåñ ³íòåãðóâàííÿ ñòåïåíåâîãî ðÿäó (12.5.7)<br />
íà ñåãìåíò³ [0, x], äå |x|