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Ïîõ³äíà çà íàïðÿìîì âåêòîðà e r â òî÷ö³ M 0 âèðàæàº<br />
øâèäê³ñòü çðîñòàííÿ ôóíêö³¿ z = f(x, y) çà òèì ñàìèì íàïðÿìîì.<br />
Çàóâàæåííÿ. ßêùî íàïðÿì âåêòîðà e r ñï³âïàäຠç<br />
äîäàòíèì íàïðÿìîì îñ³ Ox (Oy), òî ïîõ³äíà çà íàïðÿìîì e r<br />
â òî÷ö³ M 0 ïåðåòâîðþºòüñÿ â ÷àñòèííó ïîõ³äíó z′<br />
x ( x0,<br />
y0)<br />
( z′ y ( x0,<br />
y0)<br />
).<br />
Òåîðåìà 10.4.3 (ïðî ïîõ³äíó çà íàïðÿìîì). ßêùî ôóíêö³ÿ<br />
z = f(x, y) äèôåðåíö³éîâíà â òî÷ö³ M 0 (x 0 , y 0 ), òî â ö³é<br />
òî÷ö³ âîíà ìຠïîõ³äíó çà áóäü-ÿêèì íàïðÿìîì e r , ïðè öüîìó<br />
âèêîíóºòüñÿ ð³âí³ñòü<br />
∂z ∂z ∂z<br />
( x0, y0) = ( x0, y0) cos α+ ( x0, y0)<br />
sinα. ∂e ∂x ∂y<br />
(10.4.9)<br />
Ä î â å ä å í í ÿ. ßêùî ôóíêö³ÿ z = f(x, y) äèôåðåíö³éîâíà â<br />
òî÷ö³ M 0 (x 0 , y 0 ), òî â ö³é òî÷ö³ ¿¿ ïîâíèé ïðèð³ñò ìຠâèãëÿä<br />
(10.4.1).<br />
Òîä³ â³äíîøåííÿ<br />
çàïèñàòè òàê:<br />
àáî<br />
( 0,<br />
0)<br />
∆z<br />
ρ<br />
ç óðàõóâàííÿì (10.4.1) ìîæíà<br />
∆z x y ∂z ∆x ∂z ∆y ∆x ∆y<br />
= ( x , y ) + ( x , y ) +α +β<br />
x<br />
y<br />
( 0,<br />
0)<br />
0 0 0 0<br />
ρ ∂ ρ ∂ ρ ρ ρ (10.4.10)<br />
∆z x y ∂z ∂z<br />
= ( x0, y0) cos α + ( x0, y0)<br />
sinα +αcosα +βsinα, (10.4.11)<br />
ρ ∂x<br />
∂y<br />
äå<br />
limα= 0 ,<br />
∆x→0<br />
∆y→0<br />
limβ= 0 . (10.4.12)<br />
∆x→0<br />
∆→ y 0<br />
Ïåðåõ³ä â³ä ð³âíîñò³ (10.4.10) äî (10.4.11) â³ðíèé, òîìó ùî<br />
ç òðèêóòíèêà M 0 MN âèïëèâàº, ùî<br />
∆ x = cos α ∆ y<br />
, à = sin α .<br />
ρ<br />
ρ<br />
Òåïåð íåâàæêî ïåðåéòè äî ãðàíèö³ â ïðàâ³é ÷àñòèí³ ð³âíîñò³<br />
(10.4.11), êîëè ρ→0. Ãðàíèöÿ â í³é ³ñíóº, îñê³ëüêè î÷åâèäíî,<br />
ùî ïðè ρ→0 ∆x → 0 ³ ∆y → 0 (ÿêùî ã³ïîòåíóçà ïðÿìóº äî<br />
íóëÿ, òî ³ êàòåòè òåæ ïðÿìóþòü äî íóëÿ), ³ çíà÷èòü â³ðí³<br />
ð³âíîñò³ (10.4.12)). Òîä³ ³ñíóº ãðàíèöÿ â ë³â³é ÷àñòèí³ ð³âíîñò³<br />
(10.4.11) ³ ñïðàâåäëèâà ôîðìóëà (10.4.8).<br />
Òåîðåìó äîâåäåíî.<br />
Çàóâàæåííÿ 1. Àíàëîã³÷í³ òåîðåìè ìîæíà äîâåñòè ³<br />
äëÿ ôóíêö³é äèôåðåíö³éîâíèõ â òî÷ö³ Q 0 (x 0 , y 0 , z 0 ) òðèâèì³ðíîãî<br />
ïðîñòîðó (ôóíêö³ÿ u = f(x, y, z) íàçèâàºòüñÿ äèôåðåíö³éîâíîþ<br />
â òî÷ö³ Q 0 (x 0 , y 0 , z 0 ), ÿêùî â ö³é òî÷ö³ ïðèð³ñò<br />
çîáðàæóºòüñÿ ó âèãëÿä³ ∆ u = A∆ x+ B∆ y+ C∆ z+α∆ x+β∆ y+γ∆ z, äå<br />
∆x≠0, ∆y≠0, ∆z≠ 0 ³ lim ( αβγ , , ) = 0 ). Çîêðåìà, ñïðàâåäëèâà<br />
ôîðìóëà<br />
∆x→0<br />
∆y→0<br />
∆→ z 0<br />
∂z ∂z ∂z ∂z<br />
( x0, y0, z0) = ( x0, y0, z0) cos α+ ( x0, y0, z0) cos β+ ( x0, y0, z0)<br />
cosγ<br />
∂e ∂x ∂y ∂z<br />
.<br />
Òóò cos α, cos β ³ cosγ — íàïðÿìëåí³ êîñèíóñè, äëÿ ÿêèõ<br />
â³ðíà ð³âí³ñòü:<br />
2 2 2<br />
cos α+ cos β+ cos γ= 1 .<br />
Çàóâàæåííÿ 2. Ìîæíà àáñòðàêòíî ââåñòè n-âèì³ðíèé<br />
ïðîñò³ð (n > 3). Òàêå ââåäåííÿ ìè íàçâàëè àáñòðàêòíèì,<br />
îñê³ëüêè íàî÷íî âàæêî ñîá³ óÿâèòè â³äïîâ³äíó ñèñòåìó êîîðäèíàò<br />
ïðè n >3.<br />
Äëÿ n-âèì³ðíîãî ïðîñòîðó (n > 3) ââîäèòüñÿ òåæ ïîíÿòòÿ<br />
äèôåðåíö³éîâíî¿ ôóíêö³¿ u = f(x 1 , x 2 , ..., x n ) (÷èòà÷åâ³ ïðîïîíóºòüñÿ<br />
öå çðîáèòè) ³ ïîõ³äíî¿ çà íàïðÿìîì âåêòîðà<br />
e r ( cos α1,cos α2, K ,cos α n ) â òî÷ö³ M ( 0 0 0<br />
0<br />
x1, x2, K , x n ). Ïðè öüîìó<br />
ïîõ³äíà ôóíêö³¿ u = f(x 1 , x 2 , ..., x n ) â òî÷ö³ M 0 çà íàïðÿìîì<br />
âåêòîðà e r îá÷èñëþºòüñÿ çà äîïîìîãîþ òàêî¿ ôîðìóëè:<br />
∂u 0 0 ∂u 0 0 ∂u<br />
0 0<br />
( x1, K, x ) = ( x1, K, x ) cos α<br />
1<br />
+ K+ ( x1, K , x ) cosα<br />
∂e ∂x ∂x<br />
,<br />
n n n n<br />
1<br />
n<br />
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