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7.9.8. Çíàõîäèìî y′:<br />
Òîä³<br />
⎛ π ⎞<br />
y′ = cos x = sin ⎜x+<br />
⎟<br />
⎝ 2 ⎠ .<br />
⎛ π ⎞<br />
y′′ =− sin x = sin( x+π ) = sin ⎜x+ 2 ⋅ 2<br />
⎟<br />
⎝ ⎠ ,<br />
⎛ π ⎞<br />
y′′′ =− cos x = sin ⎜x+ 3 ⋅ 2<br />
⎟<br />
⎝ ⎠ ,<br />
( 4)<br />
⎛ π ⎞<br />
y = sin x = sin ( x+ 2π ) = sin ⎜x+ 4 ⋅ 2<br />
⎟<br />
⎝ ⎠ .<br />
Î÷åâèäíî, ùî äàë³ çíàõîäèòè ïîõ³äí³ íå ìຠïîòðåáè,<br />
îñê³ëüêè ¿õ âèðàçè áóäóòü öèêë³÷íî ïîâòîðþâàòèñÿ. Öå<br />
ïðèâîäèòü äî òîãî, ùî ìîæíà çàïèñàòè çàãàëüíó ôîðìóëó:<br />
( k)<br />
( sin ) ( k ) ⎛ π ⎞<br />
y = x = sin ⎜x+ k⋅ , k∈<br />
2<br />
⎟ N .<br />
⎝ ⎠<br />
Òåïåð çíàéäåìî y (k) (0):<br />
( )<br />
1 n<br />
k k ⎛ π ⎞ ⎪⎧ −<br />
(0) sin (0) sin<br />
, k = 2 n+<br />
y = x = ⎜k⋅ 1;<br />
2<br />
⎟ = ⎨<br />
⎝ ⎠ ⎪⎩ 0, k = 2 n.<br />
( ) ( ) ( )<br />
7.9.9. Àíàëîã³÷íî äîâîäèòüñÿ, ùî ìຠì³ñöå ôîðìóëà<br />
( k)<br />
( cos ) ( k ) ⎛ π ⎞<br />
y = x = cos ⎜x+ k⋅ , k∈<br />
2<br />
⎟ N .<br />
⎝ ⎠<br />
Ç îñòàííüî¿ ôîðìóëè âèïëèâຠòàêà:<br />
( )<br />
1 n<br />
k k ⎛ π ⎞ ⎪⎧ −<br />
(0) cos (0) cos<br />
, k = 2 n<br />
y = x = ⎜k⋅ ;<br />
2<br />
⎟ = ⎨<br />
⎝ ⎠ ⎪⎩ 0, k = 2n+<br />
1.<br />
( ) ( ) ( )<br />
(7.9.9)<br />
(7.9.10)<br />
7.10. ÄÈÔÅÐÅÍÖ²ÀË ÔÓÍÊÖ²¯<br />
7.10.1. Îçíà÷åííÿ<br />
Íåõàé ôóíêö³ÿ ó = f(õ) äèôåðåíö³éîâíà íà ³íòåðâàë³<br />
(a, b). Òîä³, çã³äíî ç îçíà÷åííÿì ïîõ³äíî¿, ¿¿ ïðèð³ñò ìîæíà<br />
çîáðàçèòè ó âèãëÿä³:<br />
∆ó = f′(õ) ∆õ + α(õ, ∆õ) ∆õ, (7.10.1)<br />
äå α(õ, ∆õ) ïðÿìóº äî íóëÿ ïðè ∆õ → 0.<br />
ßêùî ïðè äåÿêîìó ô³êñîâàíîìó çíà÷åííþ x: f′(õ) ≠ 0, òî<br />
ïðè ∆õ → 0 äîáóòîê f′(õ)∆õ º íåñê³í÷åííî ìàëîþ ôóíêö³ºþ<br />
îäíàêîâîãî ïîðÿäêó ìàëèçíè â³äíîñíî ∆õ (äèâ. ï. 6.2.2).<br />
Äîáóòîê æå α(õ, ∆õ) ∆õ º íåñê³í÷åííî ìàëîþ ôóíêö³ºþ<br />
á³ëüø âèñîêîãî ïîðÿäêó ìàëèçíè â³äíîñíî ∆õ, òîìó ùî<br />
α( x, ∆x)<br />
∆ x<br />
lim<br />
= lim α ( x, ∆ x) = 0.<br />
∆x<br />
∆x→0 ∆x→0<br />
Òàêèì ÷èíîì, ÿêùî f′(õ) ≠ 0, òî f′(õ)∆õ ÿâëÿº ñîáîþ ãîëîâíó<br />
÷àñòèíó ïðèðîñòó ôóíêö³¿ f(õ) ïðè ô³êñîâàíîìó õ. Öÿ<br />
÷àñòèíà ïðèðîñòó íàçèâàºòüñÿ äèôåðåíö³àëîì ôóíêö³¿ f(õ) ³<br />
ïîçíà÷àºòüñÿ dó, àáî d f(x), dó = f′(x) ∆x.<br />
Çàóâàæåííÿ. Ïîçíà÷åííÿ dó = f′(x) ∆x áóäåìî çáåð³ãàòè<br />
³ ó âèïàäêó êîëè f′(õ) = 0.<br />
ßêùî ó = õ, òî ó′ =(õ)′ = 1. Îòæå, dó = dx = ∆õ, òîáòî äèôåðåíö³àë<br />
dõ íåçàëåæíî¿ çì³ííî¿ õ çá³ãàºòüñÿ ç ¿¿ ïðèðîñòîì.<br />
Òîä³<br />
dy = f′<br />
( x)<br />
dx . (7.10.2)<br />
Çâ³äêè<br />
dy<br />
y′ = f′<br />
( x)<br />
= . (7.10.3)<br />
dx<br />
Òàêèì ÷èíîì, ïîõ³äíó f′(x) ìîæíà ðîçãëÿäàòè ÿê â³äíîøåííÿ<br />
äèôåðåíö³àëà ôóíêö³¿ äî äèôåðåíö³àëà íåçàëåæíî¿<br />
çì³ííî¿. Äèôåðåíö³àë dy = f′(x) dx íàçèâàþòü ùå äèôåðåíö³àëîì<br />
ïåðøîãî ïîðÿäêó. ²ç ôîðìóëè (7.10.2) âèïëèâàº, ùî<br />
äëÿ îá÷èñëåííÿ äèôåðåíö³àëà ôóíêö³¿ íåîáõ³äíî çíàéòè ¿¿<br />
ïîõ³äíó ³ ïîìíîæèòè íà äèôåðåíö³àë íåçàëåæíî¿ çì³ííî¿.<br />
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