Çàñòîñîâóþ÷è (n + 1) ðàç³â ôîðìóëó Êîø³ ³ âðàõîâóþ÷è âëàñòèâîñò³ (7.15.11), ìàòèìåìî Rn( x) Rn( x) − Rn( x0) R′ n( c1) + 1 + 1 ( x−x0) ( x− x0) ( n+ 1)( x−x0) ′ ( 1) − ′ ( 0) ′′( 2) n n−1 ( n+ 1)( x− x ) ( n+ 1) n( x−x ) = = = n n n ( n+ 1 ) n n n n () R c R x R c R c = = = ... = . 1! 0 0 ( n + ) (7.15.13) Ó ôîðìóë³ (7.15.13) ñòàë³ c 1 , c 2 , ..., c n , c — íåâ³äîì³, àëå çã³äíî ç òåîðåìîþ Êîø³ ìè çíàºìî, ùî âñ³ âîíè çíàõîäÿòüñÿ ì³æ õ ³ õ 0 . ßêùî x > x 0 , òî öåé ôàêò, íàïðèêëàä äëÿ ñ, ìîæíà çàïèñàòè òàê: ñ = x 0 + θ(x – x 0 ), 0 < θ
Îö³íèìî n ( ) n+ 1 θx n+ 1 x e x x R x = ≤ e 1! 1! , àëå ( n + ) ( n + ) ïðè áóäü-ÿêîìó ñê³í÷åííîìó õ, ³ òîìó Rn ( x) n+ 1 x lim = 0 n→∞ 1! ( n + ) lim = 0 . Îòæå, çà n→∞ ôîðìóëîþ (7.15.18) çíà÷åííÿ å õ ìîæíà îá÷èñëèòè ç áóäüÿêèì ñòåïåíåì òî÷íîñò³. Âðàõîâóþ÷è ôîðìóëè (7.9.9) – (7.9.10), íå âàæêî çàïèñàòè ùå äâ³ äóæå âàæëèâ³ ôîðìóëè Ìàêëîðåíà: π sin( θ x+ (2n+ 1) ) x x x sin x = x− + + ... + ( − 1) + 2 x 3! 5! (2n− 1)! (2n+ 1)! 3 5 2n−1 n− 1 2n+ 1 ( x n) x x − x cos θ +π cos x = 1 − + + ... + ( − 1) + 2! 4! (2n− 2)! (2 n)! 2 4 2n−2 n 1 2n x ; (7.15.19) . (7.15.20) 7.16. ÍÅÎÁÕ²ÄͲ ÒÀ ÄÎÑÒÀÒͲ ÓÌÎÂÈ ²ÑÍÓÂÀÍÍß ÅÊÑÒÐÅÌÓÌÓ ÔÓÍÊÖ²¯ 7.16.1. Íåîáõ³äíà óìîâà ³ñíóâàííÿ åêñòðåìóìó ôóíêö³¿ Òåîðåìà Ôåðìà ÷àñòêîâî âêàçóº íà íåîáõ³äíó óìîâó ³ñíóâàííÿ åêñòðåìóìó ôóíêö³¿. Á³ëüø çàãàëüíó òåîðåìó äîâåäåìî â öüîìó ïóíêò³. Òåîðåìà 7.16.1. ßêùî ôóíêö³ÿ y = f(x) â òî÷ö³ õ 0 ìຠëîêàëüíèé åêñòðåìóì, òî âîíà â ö³é òî÷ö³ àáî íå äèôåðåíö³éîâíà, àáî ìຠïîõ³äíó, ÿêà äîð³âíþº íóëþ. Äîâåäåííÿ. Íåõàé õ 0 òî÷êà ëîêàëüíîãî åêñòðåìóìó ³ ôóíêö³ÿ f(x) â ö³é òî÷ö³ äèôåðåíö³éîâíà. Îñê³ëüêè çíà÷åííÿ f(x 0 ) º íàéá³ëüøå àáî íàéìåíøå â îêîë³ òî÷êè õ 0 , òî çà òåîðåìîþ Ôåðìà f′(x 0 )=0. Âèïàäîê ôóíêö³¿, íå äèôåðåíö³éîâí³é ó òî÷ö³ åêñòðåìóìó, ïðî³ëþñòðóºìî íà ïðèêëàä³. ijéñíî ôóíêö³ÿ y =|x| â òî÷ö³ õ = 0 ìຠì³í³ìóì, àëå íå äèôåðåíö³éîâíà â í³é (äèâ. ïðèêë. 7.3.2). Òåîðåìà 7.16.1 ìຠïðîñòèé ãåîìåòðè÷íèé çì³ñò: ó òî÷ö³ ãðàô³êà ôóíêö³¿, ÿêà â³äïîâ³äຠòî÷ö³ ëîêàëüíîãî åêñòðåìóìó, äîòè÷íà àáî ïàðàëåëüíà îñÿì êîîðäèíàò, àáî íå ³ñíóº. Äîâåäåíà óìîâà åêñòðåìóìó º íåîáõ³äíîþ, àëå íå äîñòàòíüîþ. Íàïðèêëàä, ôóíêö³ÿ ó = õ 3 â òî÷ö³ õ = 0 ìຠïîõ³äíó, ÿêà äîð³âíþº íóëþ, àëå íå ìຠâ í³é åêñòðåìóìó. Òî÷êè, â ÿêèõ âèêîíàíà íåîáõ³äíà óìîâà åêñòðåìóìó (f′(x) = 0 àáî f(x) íå äèôåðåíö³éîâíà), íàçèâàþòüñÿ êðèòè÷íèìè. Ö³ òî÷êè “ï³äîçð³ë³” íà åêñòðåìóì. Ïèòàííÿ ïðî íàÿâí³ñòü åêñòðåìóìó â êðèòè÷íèõ òî÷êàõ âèð³øóºòüñÿ äîñòàòí³ìè óìîâàìè. Òî÷êè, â ÿêèõ f′(x) = 0, çâóòüñÿ ñòàö³îíàðíèìè. 7.16.2. Äîñòàòí³ óìîâè åêñòðåìóìó ôóíêö³¿ Òåîðåìà 7.16.2. Íåõàé äëÿ ôóíêö³¿ f(x) òî÷êà õ 0 º êðèòè÷íîþ ³ ôóíêö³ÿ f(x) äèôåðåíö³éîâíà â äåÿêîìó îêîë³ õ 0 , êð³ì, ìîæëèâî, òî÷êè õ 0 , â ÿê³é âîíà íåïåðåðâíà. Òîä³ ÿêùî ïðè ïåðåõîä³ ÷åðåç õ 0 çë³âà íàïðàâî f′(x) çì³íþº çíàê, òî ôóíêö³ÿ â òî÷ö³ õ 0 ìຠëîêàëüíèé åêñòðåìóì. Ïðè÷îìó ÿêùî çíàê f′(x) çì³íþºòüñÿ ç + íà –, òî f(x) â òî÷ö³ õ 0 ìຠëîêàëüíèé ìàêñèìóì, ÿêùî ç – íà +, òî — ëîêàëüíèé ì³í³ìóì. Äîâåäåííÿ. Ðîçãëÿíåìî âèïàäîê, êîëè çíàê ïîõ³äíî¿ f′(x) çì³íþºòüñÿ ç + íà –. Òîä³ íà â³äð³çêó ì³æ õ ³ õ 0 , äå õ — áóäü-ÿêà òî÷êà îêîëó õ 0 , äëÿ f(x) âèêîíàí³ óìîâè òåîðåìè Ëàãðàíæà. Òîìó f(x) –f(x 0 )=f′(ñ) (x – x 0 ), äå ñ∈(õ, õ 0 ). Îñê³ëüêè f′(c) > 0 ïðè x < x 0 ³ f′(c) < 0 ïðè x > x 0 , òî çàâæäè f(x) –f(x 0 ) < 0, òîáòî f(x) < f(x 0 ). Öå îçíà÷àº, ùî â òî÷ö³ õ 0 ôóíêö³ÿ f(x) ìຠëîêàëüíèé ìàêñèìóì. Àíàëîã³÷íî ðîçãëÿäàºòüñÿ âèïàäîê ëîêàëüíîãî ì³í³ìóìó. Äîâåäåíà äîñòàòíÿ óìîâà äຠïåðøèé ñïîñ³á äîñë³äæåííÿ ôóíêö³¿ íà åêñòðåìóì. Ïðèêëàä 7.16.1. Äîñë³äèòè íà åêñòðåìóì ôóíêö³þ 2 x + 2x+ 1 y= f( x) = . x −1 Ð î ç â ’ ÿ ç à í í ÿ. Ôóíêö³ÿ âèçíà÷åíà ³ äèôåðåíö³éîâíà íà ìíîæèí³ D =(–∞, 1) ∪ (1, +∞). Íà ö³é ìíîæèí³ y′ = ( x+ 1)( x−3) ( x − 1) 2 . 252 253
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