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Öå ³ º ôîðìóëà Íüþòîíà – Ëåéáí³öà, ÿêó íàçèâàþòü îñíîâíîþ<br />
ôîðìóëîþ ³íòåãðàëüíîãî ÷èñëåííÿ. ¯¿ çíà÷åííÿ âåëüìè<br />
âàæëèâå ³, ìàáóòü, íàâ³òü íåîö³íèìå, òîìó ùî âîíà âñòàíîâëþº<br />
çâ’ÿçîê ì³æ íåâèçíà÷åíèìè ³ âèçíà÷åíèìè ³íòåãðàëàìè<br />
³, êð³ì òîãî, äຠêîíñòðóêòèâíèé ìåòîä îá÷èñëåííÿ<br />
³íòåãðàë³â (áåç çíàõîäæåííÿ ãðàíèö³ â³äïîâ³äíèõ ³íòåãðàëüíèõ<br />
ñóì). Ôîðìóëà Íüþòîíà – Ëåéáí³öà º â³íöåì çóñèëü áàãàòüîõ<br />
ìàòåìàòèê³â ³ çàéìຠäîñòîéíå ì³ñöå ñåðåä øåäåâð³â<br />
ìàòåìàòè÷íî¿ äóìêè. Ùîá ï³äêðåñëèòè öå, îäèí ³ç âèäàòíèõ<br />
ðàäÿíñüêèõ ìàòåìàòèê³â Îëåêñàíäð ßêîâè÷ Õ³í÷èí (1894 –<br />
1959) ó ñâî¿õ ëåêö³ÿõ (ïåâíà ð³÷, ï³ñëÿ âèâåäåííÿ ôîðìóëè<br />
Íüþòîíà – Ëåéáí³öà) ãîâîðèâ, ùî ó ïðèñóòí³õ ñòóäåíò³â ñüîãîäí³<br />
âåëèêå ñâÿòî: âîíè îçíàéîìèëèñÿ ç îäíèì ³ç øåäåâð³â<br />
íå ò³ëüêè ìàòåìàòèêè, àëå ³ âñ³º¿ öèâ³ë³çàö³¿. ² ùîá öåé<br />
äåíü ñòàâ äëÿ ñòóäåíò³â ä³éñíî ñâÿòêîâèì, â³í, çàê³í÷óþ÷è<br />
ëåêö³þ, ïðîïîíóâàâ ¿ì â³äì³òèòè éîãî íàëåæíèì ÷èíîì.<br />
9.5. ÎÑÍÎÂͲ ÌÅÒÎÄÈ ÎÁ×ÈÑËÅÍÍß<br />
ÂÈÇÍÀ×ÅÍÈÕ ²ÍÒÅÃÐÀ˲Â<br />
Îñê³ëüêè âèçíà÷åí³ òà íåâèçíà÷åí³ ³íòåãðàëè ïîâ’ÿçàí³<br />
ì³æ ñîáîþ ôîðìóëîþ Íüþòîíà – Ëåéáí³öà, òî ìåòîäè îá÷èñëåííÿ<br />
âèçíà÷åíèõ ³íòåãðàë³â ò³ ñàì³, ùî ³ äëÿ íåâèçíà÷åíèõ<br />
³íòåãðàë³â, à ñàìå: áåçïîñåðåäí³é ìåòîä, ìåòîä çàì³íè àáî<br />
ï³äñòàíîâêè ³ ìåòîä ³íòåãðóâàííÿ ÷àñòèíàìè.<br />
9.5.1. Áåïîñåðåäí³é ìåòîä<br />
³í â îñíîâíîìó áàçóºòüñÿ íà âëàñòèâîñòÿõ 5 òà 7<br />
(ï. 9.3) ³ íà ôîðìóë³ Íüþòîíà – Ëåéáí³öà. Ñóòü öüîãî ìåòîäó<br />
ïîÿñíèìî íà ïðèêëàäàõ. Ïðè öüîìó â³äì³òèìî, ùî íà<br />
ïðàêòèö³ äëÿ çðó÷íîñò³ çàñòîñóâàííÿ ôîðìóëó (9.4.4) çàïèñóþòü<br />
òàê:<br />
Ïðèêëàäè 9.5.1 – 9.5.5.<br />
b<br />
b<br />
( ) = ( ) = ( ) − ( )<br />
∫ f x dx x b a .<br />
a<br />
9.5.1. 3 2 3 2 3<br />
3<br />
3 3<br />
∫3xdx= 3∫xdx= x = 3 − 2 = 19;<br />
2 2 2<br />
a<br />
9.5.2.<br />
4<br />
x<br />
4 4<br />
x x<br />
⎛ ⎞ 4 4<br />
4 4<br />
⎛ x ⎞<br />
4<br />
∫⎜1+ e ⎟dx = ∫dx+ 4∫e d⎜ ⎟= x + 4e = 4+ 4e− 4=<br />
4e 0⎝<br />
⎠<br />
0 0 ⎝4<br />
⎠ 0 0<br />
;<br />
dt<br />
7<br />
= + + = + = − =<br />
3 4 3 3 −1<br />
;<br />
t +<br />
3 3<br />
7 7<br />
1 1<br />
1 2 2 8<br />
−<br />
9.5.3. ∫<br />
∫( 3t 4) 2 d( 3t 4) ( 3t<br />
4) 2 ( 5 1)<br />
−1 −1<br />
9.5.4. ( )<br />
1 3<br />
1<br />
2<br />
1<br />
⎞<br />
x<br />
2 2<br />
1 1⎛<br />
2 2 4<br />
∫ x − 4x dx = ∫⎜x − 4x ⎟dx = x − 4 = − 2=−<br />
;<br />
0 0⎝<br />
⎠ 3 2 3 3<br />
0<br />
π<br />
π<br />
2 2<br />
1 1⎛<br />
1 ⎞ π<br />
∫ cos xdx = ∫ 1+ cos 2x dx = ⎜x + sin 2x<br />
⎟ = .<br />
0 2 0<br />
2⎝<br />
2 ⎠ 4<br />
2<br />
9.5.5. ( )<br />
9.5.2. Ìåòîä çàì³íè çì³ííî¿, àáî ï³äñòàíîâêè<br />
Öåé ìåòîä áàçóºòüñÿ íà òàê³é òåîðåì³.<br />
Òåîðåìà 9.5.1 (ïðî çàì³íó çì³ííî¿ ³íòåãðóâàííÿ). Íåõàé<br />
ôóíêö³ÿ f(õ) íåïåðåðâíà íà ñåãìåíò³ [à, b], à ôóíêö³ÿ<br />
õ = ϕ(t) çàäîâîëüíÿº òàêèì óìîâàì:<br />
1) ϕ(t) âèçíà÷åíà ³ íåïåðåðâíà íà ñåãìåíò³ [α, β] ³ â³äîáðàæàº<br />
ñåãìåíò [α, β] íà ñåãìåíò [à, b];<br />
2) ϕ(α) =à, ϕ(β) =b;<br />
3) ϕ(t) íåïåðåðâíî äèôåðåíö³éîâíà íà [α, β].<br />
Òîä³ ñïðàâåäëèâà ôîðìóëà:<br />
b<br />
a<br />
β<br />
( ) = ⎡ϕ( ) ⎤ϕ′<br />
( )<br />
9<br />
π<br />
2<br />
∫f x dx ∫f⎣ t ⎦ t dt. (9.5.1)<br />
α<br />
Äîâåäåííÿ. Çã³äíî ç ôîðìóëîþ Íüþòîíà — Ëåéáí³öà<br />
ìàºìî:<br />
b<br />
a<br />
( ) = ( ) − ( )<br />
∫ f x dx b a ,<br />
äå (õ) — ïåðâ³ñíà ôóíêö³¿ f(õ) íà [à, b]. Ëåãêî ïåðåêîíàòèñÿ<br />
ó òîìó, ùî ôóíêö³ÿ [ϕ(t)] º ïåðâ³ñíîþ äëÿ ôóíêö³¿<br />
f[[ϕ(t)]⋅ϕ′(t) íà [α, β]. ijéñíî, îñê³ëüêè ′(õ) =f(õ), òî ó â³äïî-<br />
0<br />
302 303