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ùî íàçèâàºòüñÿ ãðà䳺íòîì ôóíêö³¿ z = f(x, y) â òî÷ö³<br />
M 0 (x 0 , y 0 ). Òóò i r , r<br />
j â³äîì³ îðòè.<br />
Ïîìíîæèìî òåïåð ñêàëÿðíî ö³ äâà âåêòîðè, ïðè÷îìó äâîìà<br />
ñïîñîáàìè (äèâ. ï. 2.1.6). Ó ðåçóëüòàò³ îòðèìàºìî<br />
r<br />
grad z ⋅ e = z′ x ( x0, y0) ⋅cos α + z′<br />
y ( x0, y0)<br />
⋅sin<br />
α<br />
M = M<br />
, (10.5.2)<br />
0<br />
r<br />
r<br />
grad z ⋅ e = grad z ⋅ e ⋅cos<br />
ϕ<br />
M = M0 M = M<br />
,<br />
0<br />
(10.5.3)<br />
äå ϕ — êóò ì³æ öèìè âåêòîðàìè. Îñê³ëüêè<br />
r<br />
e = 1 , òî ³ç<br />
(10.5.2) – (10.5.3) áóäåìî ìàòè<br />
∂z ( x<br />
0, y<br />
0) = grad z ( x<br />
0, y<br />
0)<br />
⋅cosϕ. (10.5.4)<br />
∂e<br />
Ïðîñòèé àíàë³ç ôîðìóëè (10.5.4) ïîêàçóº, ùî íàéá³ëüøå<br />
çíà÷åííÿ ïîõ³äíî¿ çà íàïðÿìîì e r â òî÷ö³ M 0 áóäå òîä³,<br />
êîëè grad z ≠ 0 ³ íàïðÿì âåêòîðà e r ñï³âïàäຠç íàïðÿìîì<br />
ãðà䳺íòà. Ïðè öüîìó âåëè÷èíà íàéá³ëüøîãî çðîñ-<br />
M = M<br />
0<br />
òàííÿ ôóíêö³¿ z îá÷èñëþºòüñÿ çà ôîðìóëîþ<br />
∂z x y z x y z x y z x y<br />
∂e<br />
( ) ( )( ) ( ( )) 2<br />
( ( )) 2<br />
0, 0<br />
= grad<br />
0, 0<br />
= ′<br />
x 0, 0<br />
+ ′<br />
y 0,<br />
0<br />
. (10.5.5)<br />
Çàóâàæåííÿ 1. Çã³äíî ç ôîðìóëîþ (10.5.1) ãðà䳺íòîì<br />
ôóíêö³¿ z = f(x, y) â òî÷ö³ (x 0 , y 0 ) º âåêòîð, êîîðäèíàòè<br />
ÿêîãî äîð³âíþþòü â³äïîâ³äíî z′ x ( x0,<br />
y0)<br />
, z′ y ( x0,<br />
y0)<br />
.<br />
Àíàëîã³÷íî îçíà÷àºòüñÿ ãðà䳺íò ôóíêö³¿ â³ä n íåçàëåæíèõ<br />
çì³ííèõ. Ñôîðìóëþºìî, íàïðèêëàä, îçíà÷åííÿ ãðà䳺íòà<br />
äëÿ ôóíêö³¿ â³ä òðüîõ íåçàëåæíèõ çì³ííèõ.<br />
Íåõàé ôóíêö³ÿ u = f(x, y, z) â òî÷ö³ Q 0 (x 0 , y 0 , z 0 ) äèôåðåíö³éîâíà.<br />
Òîä³ â ö³é òî÷ö³ ³ñíóº ãðà䳺íò, ÿêèé âèçíà÷àºòüñÿ<br />
çà ôîðìóëîþ<br />
grad u x , y , z = u x , y , z ⋅ i r + u x , y , z ⋅ j r<br />
+ u x , y , z ⋅k,<br />
r<br />
( ) ′ ( ) ′ ( ) ′( )<br />
0 0 0 x 0 0 0 y 0 0 0 z 0 0 0<br />
äå i r , j r ³ k r çíàéîì³ íàì îðòè.<br />
Íàðåøò³ ìîæíà ñêàçàòè, ùî ïðîáëåìà ïîøóêó êëàñó<br />
ôóíêö³é (äèâ. ï. 10.3), äëÿ ÿêèõ ìîæíà ãàðàíòîâàíî çíàéòè<br />
íàïðÿì íàéá³ëüøîãî çðîñòàííÿ ôóíêö³¿ ³ ñàìó øâèäê³ñòü ¿¿<br />
çðîñòàííÿ, âèð³øåíà, öå êëàñ äèôåðåíö³éîâíèõ ôóíêö³é.<br />
Äèôåðåíö³éîâí³ ôóíêö³¿ ìàþòü äóæå âàæëèâ³ âëàñòèâîñò³.<br />
Çîêðåìà, ÿêùî ôóíêö³ÿ äèôåðåíö³éîâíà, òî âîíà íåïåðåðâíà<br />
³ ³ñíóþòü ÷àñòèíí³ ïîõ³äí³ çà áóäü-ÿêèì íàïðÿìîì. Ó<br />
çâ’ÿçêó ç öèì ïîñòຠïèòàííÿ ïðî âèÿâëåííÿ äîñòàòí³õ<br />
óìîâ, ÿê³ çàáåçïå÷óþòü äèôåðåíö³éîâí³ñòü ôóíêö³¿. ³äïîâ³äü<br />
íà çàïèòàííÿ äຠòàêà òåîðåìà.<br />
Òåîðåìà 10.5.1 (äîñòàòí³ óìîâè äèôåðåíö³éîâíîñò³<br />
ôóíêö³¿). Íåõàé ôóíêö³ÿ z = f(x, y) â äåÿêîìó δ-îêîë³ òî÷êè<br />
M 0 (x 0 , y 0 ) ìຠñê³í÷åíí³ ïîõ³äí³ z′ ( x,<br />
y)<br />
³ z ( x,<br />
y)<br />
x<br />
′ , ÿê³ íåïåðåðâí³<br />
â ñàì³é òî÷ö³ M 0 (x 0 , y 0 ). Òîä³ ôóíêö³ÿ z = f(x, y) äèôåðåíö³éîâíà<br />
â òî÷ö³ M 0 (x 0 , y 0 ).<br />
Äîâåäåííÿ. Ðîçãëÿíåìî ïîâíèé ïðèð³ñò ôóíêö³¿ â òî÷ö³,<br />
ÿêèé ìîæåìî çîáðàçèòè ó âèãëÿä³:<br />
( 0, 0) ( 0<br />
,<br />
0 ) ( 0,<br />
0)<br />
∆ zx y = fx +∆ xy +∆y − fx y =<br />
( f( x0 x, y0 y) f( x0,<br />
y0<br />
y)<br />
)<br />
= +∆ +∆ − +∆ +<br />
( f( x0, y0 y) f( x0,<br />
y0)<br />
)<br />
+ +∆ − , (10.5.6)<br />
äå ∆x ≠ 0 ³ ∆y ≠ 0.<br />
Âèðàç, ùî ñòî¿òü ó ïåðøèõ äóæêàõ, º ïðèðîñòîì ôóíêö³¿<br />
f(x, y 0 + ∆y) îäí³º¿ çì³ííî¿ x â òî÷ö³ x 0 . Îñê³ëüêè öÿ ôóíêö³ÿ<br />
äèôåðåíö³éîâíà â δ-îêîë³ òî÷êè x 0 (âíàñë³äîê ³ñíóâàííÿ<br />
÷àñòèííî¿ ïîõ³äíî¿ f′ x ( x,<br />
y)<br />
â δ-îêîë³ òî÷êè M 0 (x 0 , y 0 )), òî<br />
äî ïðèðîñòó ö³º¿ ôóíêö³¿ ìîæíà çàñòîñóâàòè òåîðåìó Ëàãðàíæà.<br />
Çã³äíî ç ö³ºþ òåîðåìîþ áóäåìî ìàòè<br />
( , ) ( , ) ′ ( , )<br />
f x +∆ x y +∆y − f x y +∆ y = f x +θ∆ x y +∆y ∆ x, (10.5.7)<br />
0 0 0 0 x 0 1 0<br />
äå 0 < θ 1