dF |
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: |
iZ RAWENSTWA |
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= 0 |
NAHODIM PROIZWODNU@ FUNKCII y(x) ZADANNOJ NEQWNO |
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tAKIM OBRAZOM, PROIZWODNU@ NEQWNOJ FUNKCII ODNOJ PEREMENNOJ MOV- NO NAHODITX, ISPOLXZUQ ^ASTNYE PROIZWODNYE FUNKCII F (x y): rASSMOTRIM PRIMER.
1: x4 ; 2 sin x + 3y3 ; ln(x ; 2y) + xy = e2y:
o^EWIDNO, ^TO FUNKCIQ |
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F (x y) = x4 ; |
2 sin x + 3y3 |
; ln(x |
; 2y) + xy ; e2y: |
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nAHODIM ^ASTNYE PROIZWODNYE \TOJ FUNKCII KAK FUNKCII DWUH NEZA- |
WISIMYH PEREMENNYH |
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1 |
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; x ; 2y |
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pODSTAWLQEM;W FORMULU; x ; 2y |
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x ; 2y + x ; 2e |
+ 9y |
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aNALOGI^NO MOVNO POLU^ITX WYRAVENIQ DLQ ^ASTNYH PROIZWODNYH NE- |
QWNOJ FUNKCII DWUH PEREMENNYH. |
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pUSTX z |
= |
z(x y) { NEQWNAQ FUNKCIQ DWUH PEREMENNYH, UDOWLETWO- |
RQ@]AQ URAWNENI@ |
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F (x y z) = 0, GDE z = z(x |
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T.E. F (x |
y z(x y)) 0: |
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tOGDA ^ASTNYE PROIZWODNYE FUNKCII z = z(x y) NAHODQTSQ S POMO]X@ |
^ASTNYH PROIZWODNYH FUNKCII F (x |
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y z) TREH NEZAWISIMYH PEREMEN- |
NYH PO FORMULAM |
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F 0 |
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rASMOTRIM PRIMER.
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2: sin(x + 3zy2) ; px3 + 7z ; |
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= 6 ln y + 2: |
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3x + 5y |
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p |
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1 |
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) ; |
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3 |
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+ 7z ; 3x |
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; 6 |
ln y ; 2: |
fUNKCIQ F (x y z) = sin(x + 3zy |
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x |
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+ 5y |
nAHODIM ^ASTNYE PROIZWODNYE \TOJ FUNKCII PO WSEM TREM NEZAWISI- |
MYM PEREMENNYM |
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F 0 |
= cos(x + 3zy2) |
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+ |
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x |
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(3x + 5y)2 |
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F 0 |
= cos(x + 3zy2) |
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3z |
2y + |
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1 |
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5 6 |
1 |
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(3x + 5y)2 |
; y |
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F 0 |
= cos(x + 3zy2) |
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3y2 |
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1 |
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7: |
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; 2px3 + 7z |
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pODSTAWLQEM POLU^ENNYE WYRAVENIQ W FORMULY DLQ ^ASTNYH PROIZ- WODNYH NEQWNOJ FUNKCII
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2 |
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3x2 |
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3 |
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F 0 |
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) |
; 2 |
x |
3 |
+ 7z |
+ |
(3x + 5y)2 |
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z0 |
= |
x = |
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7 |
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) 3y |
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z |
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) 3y |
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4.2.4. |
pROIZWODNYE WYS[IH PORQDKOW |
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pUSTX DANA FUNKCIQ z = z(x y) I PUSTX SU]ESTWU@T EE ^ASTNYE PROIZ- WODNYE @x@z I @z@y KOTORYE NAZYWA@TSQ PROIZWODNYMI PERWOGO PORQDKA.
|TI PROIZWODNYE TAKVE QWLQ@TSQ FUNKCIQMI NEZAWISIMYH PEREMEN- NYH x I y I, SLEDOWATELXNO, KAVDU@ IZ NIH MOVNO DIFFERENCIROWATX KAK PO x TAK I PO y: pOLU^AEM ^ASTNYE PROIZWODNYE WTOROGO PORQDKA
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xx |
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pROIZWODNYE |
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@2z |
I |
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NAZYWA@TSQ SME[ANNYMI PROIZWODNYMI, |
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@x@y |
@y@x |
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ONI OTLI^A@TSQ POSLEDOWATELXNOSTX@ DIFFERENCIROWANIQ. mOVNO DO-
KAZATX, ^TO, ESLI SAMA FUNKCIQ z = z(x y) I WSE EE ^ASTNYE PROIZWOD-
NYE NEPRERYWNY, TO SME[ANNYE PROIZWODNYE RAWNY MEVDU SOBOJ
@2z @2z @x@y = @y@x:
dIFFERENCIRUQ PO PEREMENNYM x I y WSE PROIZWODNYE WTOROGO PORQD- KA, BUDEM POLU^ATX PROIZWODNYE TRETXEGO PORQDKA
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z000 |
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dIFFERENCIRUQ PO x |
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ILI PO y SME[ANNYE PROIZWODNYE 2-GO PORQDKA, |
POLU^IM |
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@x @y |
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i T.P. |
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DLQ DRUGIH POSLEDOWATELXNOSTEJ WZQTIQ PROIZWODNYH. nO W ITO- |
GE POLU^IM WSEGO ^ETYRE TIPA PROIZWODNYH 3-GO PORQDKA, KOTORYE MY |
ZAPISALI. |
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z = |
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nAJTI WSE ^ASTNYE PROIZWODNYE 2-GO PORQDKA. |
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@z |
= 2(x ; y) ; |
(2x + 3y) |
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@x |
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y)2 |
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(x |
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= 0 |
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@x |
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= ;5y (;2) (x ; y);3 = |
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(x;y) (;1) = |
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(x ; y) |
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1y = 5x (x ; y);2 y0 = |
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Ax |
(x ; y) |
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5 (x |
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= ;5x ; 5y |
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(x ; y)3 |
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oTMETIM 2 O^ENX WAVNYH WYWODA: |
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{ SME[ANNYE PROIZWODNYE WTOROGO PORQDKA FUNKCII DWUH NEZAWISI- MYH PEREMENNYH WSEGDA RAWNY MEVDU SOBOJ, ESLI ONI NEPRERYWNY
{SME[ANNYE PROIZWODNYE FUNKCII L@BOGO PORQDKA RAWNY, ESLI ONI NEPRERYWNY I POLU^ENY PUTEM DIFFERENCIROWANIQ FUNKCII PO ODNIM I TEM VE PEREMENNYM ODINAKOWOE ^ISLO RAZ.
4.2.5. pOLNYJ I ^ASTNYE DIFFERENCIALY
pUSTX DANA FUNKCIQ DWUH PEREMENNYH z = z(x y). rASSMATRIWAQ ^ASTNYE PROIZWODNYE FUNKCII NESKOLXKIH PEREMENNYH, MY WWODILI PONQTIE ^ASTNYH PRIRA]ENIJ FUNKCII, KOGDA PRIRA]ENIE POLU^ALA TOLXKO ODNA PEREMENNAQ. eSLI PRIRA]ENIE POLU^A@T OBE NEZAWISIMYE PEREMENNYE, TO PRIRA]ENIE, KOTOROE PRI \TOM POLU^IT FUNKCIQ, NA- ZYWAETSQ POLNYM PRIRA]ENIEM
z = z(x + x y + y) ; z(x y):
mOVNO POKAZATX, ^TO, ESLI SU]ESTWU@T ^ASTNYE PROIZWODNYE FUNKCII z = z(x y) TO POLNOE PRIRA]ENIE FUNKCII MOVET BYTX ZAPISANO W WIDE SUMMY
z = @x@z x + @z@y y + !( x y)
GDE !( x y) - BESKONE^NO MALAQ WELI^INA BOLEE WYSOKOGO PORQDKA MALOSTI PO SRAWNENI@ S RASSTOQNIEM MEVDU TO^KAMI P (x y) I P1(x +x y + y)
o P R E D E L E N I E. gLAWNAQ, LINEJNAQ OTNOSITELXNO PRIRA]ENIJ ARGUMENTOW, ^ASTX POLNOGO PRIRA]ENIQ FUNKCII z(x y) NAZYWAETSQ POL- NYM DIFFERENCIALOM FUNKCII I OBOZNA^AETSQ dz.
iTAK, dz = @x@z x + @y@z y:
tAK KAK DIFFERENCIALY NEZAWISIMYH PEREMENNYH RAWNY IH PRIRA- ]ENIQM ( x = dx y = dy), MOVNO ZAPISATX FURMULU POLNOGO DIF- FERENCIALA W WIDE
@z @z
dz = @x dx + @y dy:
mOVNO WWESTI PONQTIE ^ASTNYH DIFFERENCIALOW : dxz = @x@z dx dyz = @z@y dy:
pOLNYJ DIFFERENCIAL RAWEN SUMME ^ASTNYH DIFFERENCIALOW dz = dxz + dyz:
wY^ISLENIE POLNOGO DIFFERENCIALA SWODITSQ K WY^ISLENI@ ^ASTNYH PROIZWODNYH I UMNOVENI@ IH NA SOOTWETSTWU@]IE DIFFERENCIALY NE- ZAWISIMYH PEREMENNYH.
1: ln(x2 + py): nAJTI dz.
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nAHODIM ^ASTNYE PROIZWODNYE |
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pODSTAWLQEM W FORMULU POLNOGO DIFFERENCIALA |
@z |
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2: u = (z + 2x) e3y: nAJTI du W TO^KE M0(1 ;1 3).
zAPI[EM FORMULU POLNOGO DIFFERENCIALA FUNKCII TREH PEREMENNYH
KAK SUMMU TREH ^ASTNYH DIFFERENCIALOW du = @u@x dx + @u@y dy + @u@z dz:
nAJDEM ^ASTNYE PROIZWODNYE |
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@x@u = 2 e3y |
@u@y = (z + 2x) e3y 3 |
@u@z = 1 e3y: |
pODSTAWLQEM W WYRAVENIE DLQ POLNOGO DIFFERENCIALA
du = e3y (2 dx + 3(z + 2x) dy + dz) :
pODSTAWLQEM KOORDINATY TO^KI M0 WMESTO x, y I z du = e;3(2 dx + 15 dy + dz):
4.2.6. dIFFERENCIALY WYS[IH PORQDKOW
dIFFERENCIAL PERWOGO PORQDKA dz FUNKCII z = z(x y) QWLQETSQ FUNKCIEJ TEH VE PEREMENNYH x I y ^TO I SAMA FUNKCIQ, PRI^EM OT x I y ZAWISQT TOLXKO WYRAVENIQ ^ASTNYH PROIZWODNYH. pO\TOMU MOV- NO NAHODITX DIFFERENCIAL OT DIFFERENCIALA PERWOGO PORQDKA, T.E. DIFFERENCIAL WTOROGO PORQDKA
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@z |
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pRIWEDEM BEZ WYWODA OKON^ATELXNOE WYRAVENIE DLQ WTOROGO DIFFEREN- |
CIALA |
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dy2. |
@x@y |
dLQ NAHOVDENIQ DIFFERENCIALA NEOBHODIMO, TAKIM OBRAZOM, NAJTI WSE ^ASTNYE PROIZWODNYE WTOROGO PORQDKA.
z = (2y + 1)cos 3x. nAJTI dz:
nAHODIM SNA^ALA ^ASTNYE PROIZWODNYE PERWOGO PORQDKA
@x@z = (2y + 1)cos 3x ln(2y + 1) (;3 sin 3x) @z@y = cos 3x (2y + 1)cos 3x;1 2:
zAPI[EM DIFFERENCIAL PERWOGO PORQDKA dz =
= (2y + 1)cos 3x ln(2y + 1) (;3 sin 3x) dx + 2 cos 3x (2y + 1)cos 3x;1 dy: nAHODIM PROIZWODNYE WTOROGO PORQDKA
@2z |
@ |
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h(2y + 1)cos 3x ln(2y + 1) (;3 sin 3x)i = |
@x2 = |
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3hx(2y+1)cos 3x |
ln(2y+1) |
(;3 sin 3x) (;3 sin 3x)+ |
+(2y + 1) |
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9 cos 3x) |
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hln(2y + 1) sin 3x ; cos 3xi |
@2z |
@ |
h2 cos 3x (2y + 1)cos 3x;1i |
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= 2 cos 3x (cos 3x ; 1) (2y + 1)cos 3x;2 2: |
@2z |
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@x@y = @y (2y + 1)cos 3x ln(2y + 1) (;3 sin 3x) =
d2z =
SOGLASNO KWADRAT,
= (;3 sin 3x) h(2y + 1)cos 3x ln(2y + 1)iy0 == |
2(2y+1)cos 3x |
= (;3 sin 3x) 22 cos 3x(2y+1)cos 3x;1 ln(2y+1)+ |
2y+1 |
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2cos 3x ln(2y + 1)+ 2y + 1 |
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cos 3x |
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pODSTAWLQEM \TI PROIZWODYE W WYRAVENIE DLQ d2z
d2z = 9 ln(2y + 1) (2y + 1)cos 3x hln(2y + 1) sin2 3x ; cos 3xi dx2+ +2 (;6 sin 3x) (2y + 1)cos 3x;1 [cos 3x ln(2y + 1) + 1] dxdy+ +4 cos 3x (cos 3x ; 1) (2y + 1)cos 3x;2 dy2:
dIFFERENCIAL WTOROGO PORQDKA MOVET BYTX ZAPISAN I W TAKOJ FORME
@x@ dx + @y@ dy!2 z
KOTOROJ SIMWOLI^ESKOE WYRAVENIE W SKOBKAH WOZWODITSQ W A ZATEM FUNKCIQ WNOSITSQ POD ZNAK WTORYH PROIZWODNYH I
POLU^AETSQ TA VE FORMULA, KOTOROJ MY UVE POLXZOWALISX. aNALOGI^NO MOVNO ZAPISATX I FORMULU WY^ISLENIQ DIFFERENNCIALA
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3-GO PORQDKA |
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4.3. pRILOVENIQ ^ASTNYH PROIZWODNYH
4.3.1. kASATELXNAQ PLOSKOSTX I NORMALX K POWERHNOSTI
pUSTX ZADANA POWERHNOSTX, OPREDELQEMAQ NEQWNYM URAWNENIEM
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F (x y : |
z) = 0 GDE z = z(x y) |
I TO^KA M0(x0 y0 |
z0) NA \TOJ POWERHNOSTI, PRI^EM FUNKCIQ |
F (x y |
z) DIFFERENCIRUEMA PO WSEM NEZAWISIMYM PEREMENNYM W TO^KE |
M0(x0 |
y0 z0). dADIM OPREDELENIE KASATELXNOJ PLOSKOSTI. |
o P R E D E L E N I E. kASATELXNOJ PLOSKOSTX@ K DANNOJ POWERHNOSTI W TO^KE M0(x0 y0 z0) NAZYWAETSQ PLOSKOSTX, W KOTOROJ LEVAT WSE KA- SATELXNYE, PROWEDENNYE K L@BOJ KRIWOJ, PRINADLEVA]EJ POWERHNOSTI I PROHODQ]EJ ^EREZ \TU TO^KU.
Fx0(M0) (x ; x0) + Fy0(M0) (y ; y0) + Fz0(M0) (z ; z0) = 0
uRAWNENIE KASATELXNOJ PLOSKOSTI IMEET WID
kOORDINATAMI WEKTORA NORMALI KASATELXNOJ PLOSKOSTI QWLQ@TSQ ZNA-
^ENIQ ^ASTNYH PROIZWODNYH FUNKCII F (x y z) W TO^KE M0(x0 |
y0 z0) : |
~ |
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F |
0(M0) |
F 0(M0) |
F |
0(M0) |
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eSLI POWERHNOSTX ZADANA W QWNOM WIDE z = z(x y) TO WEKTOR NORMALI |
KASATELXNOJ PLOSKOSTI BUDET IMETX KOORDINATY |
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I URAWNENIE KASATELXNOJ PLOSKOSTI |
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o P R E D E L E N I E. |
nORMALX@ |
K POWERHNOSTI W TO^KE M0(x0 |
y0 z0) |
NAZYWAETSQ PRQMAQ, PROHODQ]AQ ^EREZ TO^KU KASANIQ, PERPENDIKULQRNO KA- |
SATELXNOJ PLOSKOSTI. |
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nAPRAWLQ@]IM WEKTOROM NORMALI QWLQETSQ WEKTOR NORMALI KASA-
TELXNOJ PLOSKOSTI. nORMALX K POWERHNOSTI, ZADANNOJ NEQWNO, OPRE- DELQETSQ URAWNENIEM
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x ; x0 |
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A W SLU^AE QWNOGO ZADANIQ POWERHNOSTI URAWNENIE NORMALI |
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sOSTAWITX URAWNENIE KASATELXNOJ PLOSKOSTI I NORMALI K POWERH- |
NOSTI 3x2 + 5y2 + 2z2 = 41 W TO^KE M0(1 2 3): |
nAHODIM ^ASTNYE PROIZWODNYE FUNKCII |
F (x y z) = 3x |
2 |
+ 5y |
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+ 2z |
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; 41 |
I WY^ISLQEM IH W TO^KE |
M0: |
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tAKIM OBRAZOM, WEKTOR NORMALI IMEET KOORDINATY |
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N = f6 20 12g |
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zAPISYWAEM URAWNENIE KASATELXNOJ PLOSKOSTI |
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3 (x ;1) + 10 (y ;2) +6 (z ;3) = 0 ) 3x + 10y + 6z ;41 = 0: |
uRAWNENIE NORMALI |
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1 = y ; 2 |
= z ; 3: |
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4.3.2. |KSTREMUM FUNKCII DWUH PEREMENNYH
pUSTX FUNKCIQ z = f(x y) OPREDELENA W NEKOTOROJ OBLASTI (D), A
M0(x0 y0) - WNUTRENNQQ TO^KA \TOJ OBLASTI.
o P R E D E L E N I E. tO^KA M0(x0 y0) NAZYWAETSQ TO^KOJ MAK-
SIMUMA FUNKCII z = |
f(x |
y), |
ESLI ZNA^ENIE FUNKCII W \TOJ TO^KE |
QWLQETSQ NAIBOLX[IM ZNA^ENIEM FUNKCII W NEKOTOROJ OKRESTNOSTI TO^KI |
M0(x0 y0) : |
f(x0 y0) f(x y): |
o P R E D E L E N I E. |
tO^KA M0(x0 y0) NAZYWAETSQ TO^KOJ MINI- |
MUMA FUNKCII z = f(x y), |
ESLI ZNA^ENIE FUNKCII W \TOJ TO^KE QW- |
LQETSQ NAIMENX[IM ZNA^ENIEM FUNKCII W NEKOTOROJ OKRESTNOSTI TO^KI |
M0(x0 y0): |
f(x0 y0) f(x y): |
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oTMETIM, ^TO W OKRESTNOSTI TO^EK \KSTREMUMA POLNOE PRIRA]ENIE |
FUNKCII SOHRANQET SWOJ ZNAK. |
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nEOBHODIMYJ PRIZNAK \KSTREMUMA |
dLQ TOGO, ^TOBY FUNKCIQ z = f(x |
y) IMELA W TO^KE M0(x0 y0) \KSTRE- |
MUM, NEOBHODIMO, ^TOBY EE ^ASTNYE PROIZWODNYE PERWOGO PORQDKA LIBO |
RAWNQLISX NUL@, LIBO NE SU]ESTWOWALI W \TOJ TO^KE |
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8 |
@x@z !M0 |
= 0 |
(1 |
NE |
SU]ESTWUET ) |
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> |
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(?) |
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@y@z ! |
= 0 |
(1 |
NE |
SU]ESTWUET:) |
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, : , - tO^KI W KOTORYH WYPOLNQ@TSQ NEOBHODIMYE USLOWIQ \KSTREMUMA NA
ZYWA@TSQ KRITI^ESKIMI TO^KAMI FUNKCII I TOLXKO W NIH FUNKCIQ MOVET PRINIMATX \KSTREMALXNYE ZNA^ENIQ.
dOSTATO^NYE USLOWIQ \KSTREMUMA
pUSTX W TO^KE M0(x0 y0) WYPOLNENY NEOBHODIMYE USLOWIQ \KSTREMUMA (?): nAJDEM WSE WTORYE ^ASTNYE PROIZWODNYE FUNKCII I OBOZNA^IM IH ZNA^ENIQ W \TOJ TO^KE
a11 |
= 0 |
@2z |
1 |
a12 |
= 0 |
@2z |
1 |
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a22 |
= 0 |
@2z |
1 |
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AM0 |
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151 |
sFORMULIRUEM DOSTATO^NYE USLOWIQ \KSTREMUMA.
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eSLI |
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TO FUNKCIQ IMEET W TO^KE |
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a11a22 ; a12 > 0 |
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M0(x0 y0) \KSTREMUM PRI^EM : |
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ESLI |
a11 < 0 |
; W |
TO^KE |
max |
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ESLI |
a11 > 0 ; W |
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min: |
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2: eSLI |
a11a22 ; a122 < 0 TO FUNKCIQ NE IMEET W TO^KE |
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M0(x0 y0) \KSTREMUMA: |
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3: eSLI |
a11a22 ; a122 |
= 0 TO WOPROS O SU]ESTWOWANII |
\KSTREMUMA NE RE[EN TREBU@TSQ DOPOLNITELXNYE ISSLEDOWANIQ:
zADA^A 5. iSSLEDOWATX NA \KSTREMUM FUNKCII
1: z = x2 + xy + y2 ; 4x + y ; 6:
1)fUNKCIQ OPREDELENA DLQ WSEH ZNA^ENIJ PEREMENNYH x I y:
2)nAHODIM ^ASTNYE PROIZWODNYE PERWOGO PORQDKA I SOSTAWLQEM SISTE- MU DLQ OPREDELENIQ KOORDINAT TO^EK, W KOTORYH WOZMOVEN \KSTREMUM
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= 2x + y 4 |
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z0 |
= x + 2y |
+ 1: |
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z0 = 0 |
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;2x + y 4 = 0 |
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x = 3 |
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8 z0 = 0 |
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8 y = 2 |
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8 x + 2y + 1 = 0 |
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tAKIM OBRAZOM, |
NAJDENA KRITI^ESKAQ (ILI STACIONARNAQ) TO^KA |
M0(3 |
;2): |
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3) nAHODIM ^ASTNYE PROIZWODNYE WTOROGO PORQDKA I WY^ISLQEM IH W |
TO^KE |
M0(3 ;2) |
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a11 = 0 |
@2z |
1 |
= 2 a12 |
= 0 |
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@2z |
1 |
= 1 a22 |
= 0 |
@2z |
1 |
= 2: |
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2 |
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2 |
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@ |
@x |
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AM0 |
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@x@yAM0 |
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@y |
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AM0 |
w DANNOM SLU^AE KOORDINATY TO^KI NE PONADOBILISX, TAK KAK PROIZ- |
WODNYE WTOROGO PORQDKA { POSTOQNNYE ^ISLA. |
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sOSTAWLQEM WYRAVENIE a11a22 |
; a122 = 2 2 ; 12 = 3 > 0: |
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oTME^AEM, ^TO \KSTREMUM SU]ESTWUET I, TAK KAK, PRI \TOM a11 > 0 |
{ W TO^KE M0(3 |
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zNA^ENIE FUNKCII W TO^KE: zmin = z(3 ;2) = ;13: |
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152 |
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