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10.4. ÄÈÔÅÐÅÍÖ²ÉÎÂͲÑÒÜ ÔÓÍÊÖ²¯<br />
10.4.1. Äèôåðåíö³éîâí³ñòü â òî÷ö³<br />
Íåõàé ôóíêö³ÿ z = f(x, y) âèçíà÷åíà â δ-îêîë³ òî÷êè<br />
M 0 (x 0 , y 0 ). ³çüìåìî òî÷êó M(x 0 + ∆x, y 0 + ∆y) ç öüîãî îêîëó.<br />
гçíèöÿ ∆z = f(x 0 + ∆x, y 0 + ∆y) –f(x 0 , y 0 ) ïðè ∆x ≠ 0 ³ ∆y ≠ 0<br />
íàçèâàºòüñÿ ïîâíèì ïðèðîñòîì ö³º¿ ôóíêö³¿ â òî÷ö³<br />
M 0 (x 0 , y 0 ).<br />
Îçíà÷åííÿ 10.4.1. Ôóíêö³ÿ z = f(x, y), ÿêà âèçíà÷åíà â<br />
îêîë³ òî÷êè M 0 (x 0 , y 0 ), íàçèâàºòüñÿ äèôåðåíö³éîâíîþ â òî÷ö³,<br />
ÿêùî ¿¿ ïîâíèé ïðèð³ñò ó ö³é òî÷ö³ ìîæíà çîáðàçèòè ó<br />
âèãëÿä³:<br />
( 0,<br />
0)<br />
∆ zx y = A⋅∆ x+ B⋅∆ y+α⋅∆ x+β⋅∆ y, (10.4.1)<br />
äå ÷èñëà A ³ B íå çàëåæàòü â³ä ∆x ³ ∆y, à<br />
limα= 0 ,<br />
∆x→0<br />
∆y→0<br />
limβ= 0 .<br />
∆x→0<br />
∆→ y 0<br />
∆x→0<br />
∆y→0<br />
Äëÿ äèôåðåíö³éîâíèõ ôóíêö³é äîâîäèòüñÿ íèçêà âàæëèâèõ<br />
òåîðåì.<br />
Òåîðåìà 10.4.1 (ïðî íåïåðåðâí³ñòü). ßêùî ôóíêö³ÿ<br />
z = f(x, y) äèôåðåíö³éîâíà â òî÷ö³ M 0 (x 0 , y 0 ), òî âîíà â ö³é<br />
òî÷ö³ íåïåðåðâíà.<br />
ijéñíî, ÿêùî ôóíêö³ÿ z = f(x, y) äèôåðåíö³éîâíà â òî÷ö³<br />
M 0 (x 0 , y 0 ), òî ¿¿ ïîâíèé ïðèð³ñò çã³äíî ç îçíà÷åííÿì ìàº<br />
âèãëÿä (10.4.1).<br />
Ïåðåõîäÿ÷è äî ãðàíèö³ â ð³âíîñò³ (10.4.1) ïðè ∆x<br />
→ 0 ³<br />
∆y<br />
→ 0, áóäåìî ìàòè, ùî lim∆ z = 0 . À öå îçíà÷àº, ùî ôóíêö³ÿ<br />
z = f(x, y) íåïåðåðâíà â òî÷ö³ M 0 (x 0 , y 0 ).<br />
Òåîðåìó äîâåäåíî.<br />
Òåîðåìà 10.4.2 (ïðî ³ñíóâàííÿ ÷àñòèííèõ ïîõ³äíèõ).<br />
ßêùî ôóíêö³ÿ z = f(x, y) äèôåðåíö³éîâíà â òî÷ö³ M 0 (x 0 , y 0 ),<br />
òî â ö³é òî÷ö³ ³ñíóþòü ÷àñòèíí³ ïîõ³äí³ z′ x ( x0,<br />
y0)<br />
³ z′ y ( x0,<br />
y0)<br />
,<br />
ïðè÷îìó, ÿêùî ïîâíèé ïðèð³ñò ôóíêö³¿ çàïèñàíèé ó âèãëÿä³<br />
(10.4.1), òî<br />
( , ) , ( , )<br />
z′ x y = A z′<br />
x y = B.<br />
x<br />
0 0 y 0 0<br />
Äîâåäåííÿ. ßêùî ôóíêö³ÿ z = f(x, y) äèôåðåíö³éîâíà â<br />
òî÷ö³ M 0 (x 0 , y 0 ), òî ¿¿ ïîâíèé ïðèð³ñò ó ö³é òî÷ö³ çã³äíî ç<br />
îçíà÷åííÿì çàïèøåìî ó âèãëÿä³ (10.4.1). Íåõàé òåïåð<br />
∆y = 0, à ∆x ≠ 0. Òîä³ ïîâíèé ïðèð³ñò ôóíêö³¿ â òî÷ö³<br />
M 0 (x 0 , y 0 ) áóäå ñï³âïàäàòè ç ÷àñòèííèì ïðèðîñòîì ïî x<br />
ö³º¿ ôóíêö³¿ â òî÷ö³ M 0 (x 0 , y 0 ), òîáòî ð³âí³ñòü (10.4.1) ìîæíà<br />
çàïèñàòè ó âèãëÿä³:<br />
äå<br />
limα= 0 . Çâ³äñè<br />
∆x→0<br />
( 0,<br />
0)<br />
∆ xz x y = A⋅∆ x+α⋅∆ x,<br />
∆ xz = A +α .<br />
∆x<br />
Ïðè ∆x → 0 ïðàâà ÷àñòèíà îñòàííüî¿ ð³âíîñò³ ïðÿìóº äî<br />
÷èñëà A, öå îçíà÷àº, ùî ôóíêö³ÿ z = f(x, y) â òî÷ö³ M 0 (x 0 , y 0 )<br />
ìຠ÷àñòèííó ïîõ³äíó (ñê³í÷åííó) z′ x ( x0,<br />
y0)<br />
³ ñïðàâäæóºòüñÿ<br />
ð³âí³ñòü<br />
z′ x , y = A. (10.4.2)<br />
x<br />
( )<br />
0 0<br />
ßêùî ∆x = 0, à ∆y ≠ 0, òî, ïðîâîäÿ÷è àíàëîã³÷í³ ïåðåòâîðåííÿ,<br />
ïåðåêîíàºìîñÿ, ùî ÷àñòèííà ïîõ³äíà ïî y â³ä ôóíêö³¿<br />
z = f(x, y) ó òî÷ö³ M 0 (x 0 , y 0 ) òàêîæ ³ñíóº ³ ñïðàâåäëèâà ð³âí³ñòü<br />
y<br />
( 0,<br />
0)<br />
z′ x y = B. (10.4.3)<br />
Òåîðåìó äîâåäåíî.<br />
10.4.2. Äèôåðåíö³àë ôóíêö³¿<br />
Íàãàäàºìî, ùî ÿêùî ôóíêö³ÿ z = f(x, y) äèôåðåíö³éîâíà â<br />
òî÷ö³ M 0 (x 0 , y 0 ), òî ¿¿ ïîâíèé ïðèð³ñò ìîæå áóòè çîáðàæåíèé<br />
ó âèãëÿä³:<br />
( , )<br />
∆ zx y = A⋅∆ x+ B⋅∆ y+α⋅∆ x+β⋅∆ y, (10.4.4)<br />
0 0<br />
äå ÷èñëà A ³ B íå çàëåæàòü â³ä ∆x ³ ∆y, à<br />
limα= 0 ,<br />
∆x→0<br />
∆y→0<br />
limβ= 0 .<br />
∆x→0<br />
∆→ y 0<br />
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