Create successful ePaper yourself
Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.
Öåé æàðò³âëèâèé åêñïåðèìåíò äîïîì³ã ñòóäåíòîâ³ çðîçóì³òè<br />
ïàðàäîêñàëüíèé íà ïåðøèé ïîãëÿä ôàêò: ÷àñòèíí³<br />
ïîõ³äí³ ôóíêö³¿ â äàí³é òî÷ö³ ìîæóòü ³ñíóâàòè, à ñàìà<br />
ôóíêö³ÿ â í³é ðîçðèâíà.<br />
ÂÏÐÀÂÈ<br />
10.6. Çíàéòè ÷àñòèíí³ ïîõ³äí³ ôóíêö³¿ z = x y â òî÷ö³<br />
M 0 (1, 2).<br />
10.7. Çíàéòè ÷àñòèíí³ ïîõ³äí³ ôóíêö³¿ z = arctg x y â òî÷ö³<br />
M 0 (1, 1).<br />
10.8. Çíàéòè ÷àñòèíí³ ïîõ³äí³ ôóíêö³¿ z = sin( x 2 + y<br />
2<br />
) â<br />
òî÷ö³ M 0 (0, 1).<br />
∂z<br />
∂z<br />
1<br />
10.9. Äîâåñòè, ùî x + y = , äå z = ln ( x + y)<br />
.<br />
∂x<br />
∂y<br />
2<br />
∂z<br />
∂z<br />
3 2 3<br />
10.10. Äîâåñòè, ùî x + y = 3z, äå z = x + xy − 2y<br />
.<br />
∂x<br />
∂y<br />
10.3.2. Ïîõ³äí³ âèùèõ ïîðÿäê³â<br />
2<br />
Íåõàé ôóíêö³ÿ z = f(x, y) ìຠâ îáëàñò³ D ⊂ R ÷àñòèíí³<br />
ïîõ³äí³ z′ x ( x,<br />
y)<br />
³ z′ y ( x,<br />
y)<br />
. Ìè íàçèâàòèìåìî ¿õ ïîõ³äíèìè<br />
ïåðøîãî ïîðÿäêó. Ö³ ïîõ³äí³, ÿê³ ðîçãëÿäàþòüñÿ ÿê ôóíêö³¿<br />
â³ä x ³ y, â ñâîþ ÷åðãó ìîæóòü ìàòè ÷àñòèíí³ ïîõ³äí³ ïî x<br />
³ y. Òîáòî º ñåíñ ðîçãëÿäàòè ïîõ³äí³ ( z ′<br />
x )<br />
′<br />
x<br />
, ( z ′ x )<br />
′ , ( )<br />
y<br />
z ′ ′ ³<br />
y x<br />
( z ′<br />
y )<br />
′<br />
y<br />
,<br />
ÿê³ ìè â³äïîâ³äíî ïîçíà÷èìî òàê:<br />
ðÿäîê çì³øàíèõ ïîõ³äíèõ äîð³âíþº 2, à ñàìå ïîõ³äí³ òàê³:<br />
z′′<br />
xy ³<br />
z′′<br />
yx . ßñíî, ùî ç³ çðîñòàííÿì n ¿õ ê³ëüê³ñòü çíà÷íî<br />
çðîñòàº. ßêùî ìîæëèâî äèôåðåíö³þâàòè n ðàç³â ïî çì³ííèõ<br />
x ³ y, òî â ðåçóëüòàò³ áóäåìî ìàòè 2 n ïîõ³äíèõ n-ãî ïîðÿäêó.<br />
Ïîêàæåìî ñõåìàòè÷íî ïðîöåñ çíàõîäæåííÿ ïîõ³äíèõ âèùèõ<br />
ïîðÿäê³â:<br />
x<br />
Ïðèêëàä 10.3.1. Íåõàé z = e cos y . Òðåáà çíàéòè óñ³ ïîõ³äí³<br />
äðóãîãî ïîðÿäêó ³ ïîêàçàòè, ùî:<br />
z′′ = z′′<br />
;<br />
1) xy yx<br />
2) z′′ + z′′<br />
= 0 .<br />
xx<br />
yy<br />
Ðîçâ’ÿçàííÿ<br />
′ ′ ′ ′<br />
z′ = z′′ z′ = z′′ z′ = z′′ z′ = z′′<br />
. (10.3.5)<br />
( ) , ( ) , ( ) , ( )<br />
x x xx x y xy y<br />
x<br />
yx y<br />
y<br />
yy<br />
Îòðèìàí³ ïîõ³äí³ íàçèâàþòüñÿ ïîõ³äíèìè äðóãîãî ïîðÿäêó.<br />
¯õ âñüîãî ÷îòèðè. ßêùî ïðîöåñ ïîäàëüøîãî äèôåðåíö³þâàííÿ<br />
ìîæëèâèé, òî ïðè äèôåðåíö³þâàíí³ ôóíêö³é, âèçíà-<br />
÷åíèõ çà ôîðìóëîþ (10.3.5), ìè îòðèìàºìî âæå 8 ïîõ³äíèõ<br />
òðåòüîãî ïîðÿäêó ³ ò. ä. Ñåðåä íèõ áóäóòü çóñòð³÷àòèñÿ ïîõ³äí³<br />
òèïó z′′<br />
xx<br />
, à òàêîæ òèïó z′′<br />
xy . Ïîõ³äí³ îñòàííüîãî òèïó<br />
áóäåìî íàçèâàòè çì³øàíèìè ïîõ³äíèìè. ßêíàéìåíøèé ïî-<br />
Îòæå, áåçïîñåðåäíüî âñòàíîâëþºìî, ùî ä³éñíî ñïðàâåäëèâà<br />
òîòîæí³ñòü 1).<br />
x<br />
x<br />
2) Îñê³ëüêè, z′′ xx<br />
= e cos y, à z′′ yy<br />
=− e cos y, òî ð³âí³ñòü 2) òåæ<br />
ñïðàâäæóºòüñÿ.<br />
Çàóâàæåííÿ. Òîòîæí³ñòü òèïó 1) íå âèïàäêîâà. Ç íåþ<br />
ïîâ’ÿçàíà.<br />
360 361