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Çàóâàæèìî, ùî ïîíÿòòÿ åêñòðåìóìó ôóíêö³¿ íîñèòü ëîêàëüíèé<br />
õàðàêòåð. ̳í³ìóì ôóíêö³¿ ìîæå áóòè á³ëüøå ìàêñèìóìó<br />
(ðèñ. 10.15).<br />
Ðèñ. 10.15<br />
Òåîðåìà 10.6.1 (íåîáõ³äíà óìîâà åêñòðåìóìó ôóíêö³¿<br />
äâîõ íåçàëåæíèõ çì³ííèõ). ßêùî ôóíêö³ÿ z = f(x, y) â òî÷ö³<br />
M 0 (x 0 , y 0 ) ìຠåêñòðåìóì, òî â ö³é òî÷ö³ ÷àñòèíí³ ïîõ³äí³ íå<br />
³ñíóþòü àáî äîð³âíþþòü íóëþ.<br />
Ä î â å ä å í í ÿ. Íåõàé â òî÷ö³ M 0 (x 0 , y 0 ) ôóíêö³ÿ z = f(x, y)<br />
ìຠåêñòðåìóì, òîä³ ³ ôóíêö³¿ ϕ = f(x, y 0 ), ψ = f(x 0 , y) ìàþòü<br />
åêñòðåìóìè â³äïîâ³äíî â òî÷êàõ x 0 ³ y 0 . Òàêèì ÷èíîì (äèâ.<br />
ï. 7.13.1), ïîõ³äí³ öèõ ôóíêö³é ó äàíèõ òî÷êàõ àáî íå ³ñíóþòü,<br />
àáî äîð³âíþþòü íóëþ:<br />
( x ) z ( x y )<br />
ϕ ′ = ′ , = 0,<br />
0 x 0 0<br />
( y ) z ( x y )<br />
ψ ′ = ′ , = 0.<br />
0 y 0 0<br />
Òåîðåìó äîâåäåíî.<br />
Òî÷êè, â ÿêèõ ÷àñòèíí³ ïîõ³äí³ íå ³ñíóþòü àáî äîð³âíþþòü<br />
íóëþ, íàçèâàþòüñÿ êðèòè÷íèìè, àáî “òî÷êàìè ï³äîçð³ëèìè<br />
íà åêñòðåìóì”.<br />
Çàóâàæåííÿ 1. Òî÷êè, â ÿêèõ âèêîíóþòüñÿ óìîâè<br />
( x y)<br />
z′ , = 0, (10.6.1)<br />
x<br />
( x y)<br />
z′ , = 0, (10.6.2)<br />
y<br />
íàçèâàþòüñÿ ñòàö³îíàðíèìè. ¯õ òàêîæ ìîæíà íàçèâàòè êðèòè÷íèìè<br />
òî÷êàìè, îñê³ëüêè âîíè ÿâëÿþòü ñîáîþ ï³äìíîæèíó<br />
ìíîæèíè êðèòè÷íèõ òî÷îê. Äëÿ çíàõîäæåííÿ ñòàö³îíàðíèõ<br />
òî÷îê ð³âíÿííÿ (10.6.1) – (10.6.2) ðîçãëÿäàþòüñÿ ÿê<br />
ñèñòåìà.<br />
Çàóâàæåííÿ 2. Ó âèïàäêó ñòàö³îíàðíèõ òî÷îê ñôîðìóëüîâàíà<br />
òåîðåìà º äâîâèì³ðíèì àíàëîãîì òåîðåìè Ôåðìà.<br />
Çàóâàæåííÿ 3. Äëÿ çíàõîäæåííÿ ñòàö³îíàðíèõ òî÷îê<br />
äèôåðåíö³éîâíî¿ ôóíêö³¿ n çì³ííèõ òðåáà ðîçâ’ÿçóâàòè ñèñòåìó<br />
n ð³âíÿíü, ÿêà àíàëîã³÷íà ñèñòåì³ (10.6.1) – (10.6.2)<br />
(óñ³ ÷àñòèíí³ ïîõ³äí³ ïåðøîãî ïîðÿäêó òàêî¿ ôóíêö³¿ â ñòàö³îíàðí³é<br />
òî÷ö³ äîð³âíþþòü íóëþ).<br />
Çàçíà÷èìî òåïåð, ùî óìîâè (10.6.1) – (10.6.2) àáî óìîâè<br />
íå ³ñíóâàííÿ ÷àñòèííèõ ïîõ³äíèõ º ò³ëüêè íåîáõ³äíèìè.<br />
Ùîá ó öüîìó ïåðåêîíàòèñÿ ðîçãëÿíåìî ïðèêëàäè.<br />
Ïðèêëàä 10.6.1. z = x 2 – y 2 .<br />
Ãðàô³ê ö³º¿ ôóíêö³¿ (ïîâåðõíÿ) çîáðàæåíî íà ðèñ. 10.16.<br />
Ðèñ. 10.16<br />
Ïîâåðõíÿ ôóíêö³¿ z = x 2 – y 2 íàãàäóº ñ³äëî. Äîñë³äèìî ¿¿<br />
íà åêñòðåìóì. Ñïî÷àòêó çíàéäåìî ñòàö³îíàðí³ òî÷êè:<br />
z′ = 2x= 0⇒ x = 0<br />
x<br />
z′ =− 2y = 0⇒ y = 0.<br />
y<br />
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