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А.Н.Шерстнев - Математический анализ
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eSLI VE IMEET MESTO OBRATNOE NERAWENSTWO f ( x + (1 , )y) f(x) + (1 , )f (y), GOWORQT, ^TO FUNKCIQ WOGNUTA (WYPUKLA WWERH).
2. gEOMETRI^ESKI USLOWIE ( ) OZNA^AET, ^TO MNOVESTWO
E = f(x; y) 2 R2 j x 2 (a; b); f(x) yg
QWLQETSQ WYPUKLYM, TO ESTX WMESTE S KAVDYMI SWOIMI DWUMQ TO^KAMI ONO SODERVIT I OTREZOK, SOEDINQ@]IJ \TI TO^KI.
3. dIFFERENCIRUEMAQ FUNKCIQ f(x) (a < x < b) NAZYWAETSQ WYPUKLOJ (SOOTWETSTWENNO WOGNUTOJ) W TO^KE c 2 (a; b), ESLI W NEKOTOROJ OKREST- NOSTI TO^KI c GRAFIK \TOJ FUNKCII NAHODITSQ NAD (SOOTWETSTWENNO POD) KASATELXNOJ W TO^KE c. gOWORQT, ^TO c | TO^KA PEREGIBA, ESLI DLQ NEKO- TOROGO > 0 W INTERWALAH (c, ; c); (c; c+ ) GRAFIK NAHODITSQ PO RAZNYE STORONY OT KASATELXNOJ W TO^KE c. pRIWEDEM• PRAKTI^ESKI \FFEKTIWNYE USLOWIQ WYPUKLOSTI FUNKCII.
4. dIFFERENCIRUEMAQ NA (a; b) FUNKCIQ f WYPUKLA TTOGDA f0 NE UBY- |
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WAET NA (a; b). w ^ASTNOSTI, ESLI f DWAVDY DIFFERENCIRUEMA NA (a; b), |
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TO ONA WYPUKLA TTOGDA |
f00(x) |
0 (a < x < b). |
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pUSTX WYPUKLAQ FUNKCIQ f DIFFERENCIRUEMA NA (a; b); a < x < y < b |
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I h > 0 TAKOWO, ^TO x + h < y. pOLAGAQ = 1 , y , x , IMEEM x + h = |
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x + (1 , )y I, SLEDOWATELXNO, |
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[ f(x) + (1 , )f (y) |
, f (x)] |
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(x): |
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oTS@DA f0(x) = |
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f(x)] |
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f(y) , f(x). aNALOGI^NYE WY- |
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h!0+ h |
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y , x |
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f(y |
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^ISLENIQ DLQ > 0 TAKOGO, ^TO x < y, , POKAZYWA@T, ^TO |
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f (y) , f (x), TAK ^TO |
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f0(y) = |
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f(y) , f(x) |
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nEOBHODIMOSTX PERWOGO UTWERVDENIQ DOKAZANA.
71
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pUSTX TEPERX f0 NE UBYWAET NA (a; b); a < x < y < b I z = x + (1 , )y; 2 [0; 1]. pRIMENQQ FORMULU lAGRANVA, POLU^AEM
f (x) , f(z) = f0( )(x , z); f(y) , f(z) = f0( )(y , z);
GDE 2 (x; z); 2 (z; y), TAK ^TO f0( ) f0( ). sLEDOWATELXNO,
f(x) + (1 , )f(y) , f ( x + (1 , )y)
= (f (x) , f(z)) + (1 , )(f (y) , f (z)) = f0( )(x , z) + (1 , )f0 ( )(y , z)f0 ( )[ (x , z) + (1 , )(y , z)] = 0:
dOSTATO^NOSTX USTANOWLENA. ~ASTNOE UTWERVDENIE SLEDUET TEPERX IZ TAB-
LICY 38.1. >
5. pUSTX f00 OPREDELENA W NEKOTOROJ OKRESTNOSTI U (c) I NEPRERYWNA W TO^KE c. tOGDA
(A) f00(c) > 0 WLE^•ET, ^TO f WYPUKLA W TO^KE c,
(B) f00(c) < 0 WLE^•ET, ^TO f WOGNUTA W TO^KE c,
(W) ESLI f00(c) = 0 I f(3) OPREDELENA W NEKOTOROJ OKRESTNOSTI U (c), NEPRERYWNA W TO^KE c I f(3) (c) 6= 0; TO c | TO^KA PEREGIBA.
uTWERVDENIQ (A) I (B) SLEDU@T IZ PREDSTAWLENIQ
GDE r2(x) = 12(x , c)2f00(c + (x , c)) HARAKTERIZUET PREWY[ENIE GRAFIKA NAD KASATELXNOJ y = f (c)+f0 (c)(x,c) W TO^KE c. eSLI, NAPRIMER, f00(c) > 0, TO W SILU NEPRERYWNOSTI f00 W TO^KE c FUNKCIQ f00 SOHRANQET ZNAK W NEKOTOROJ OKRESTNOSTI TO^KI c, I ZNA^IT, W \TOJ OKRESTNOSTI GRAFIK NAHODITSQ NAD KASATELXNOJ, TO ESTX f WYPUKLA W TO^KE c.
w SLU^AE (W)
f (x) = f(c) + f0 (c)(x , c) + 3!1 (x , c)3f(3) (c + (x , c));
I SNOWA ZAMETIM, ^TO f(3) SOHRANQET ZNAK W NEKOTOROJ OKRESTNOSTI TO^KI c, A SOMNOVITELX (x , c)3 MENQET ZNAK PRI PEREHODE ^EREZ TO^KU c: >
6. p R I M E R. fUNKCIQ f(x) = xb (x > 0) WYPUKLA PRI b 1, TAK KAK f00(x) = b(b , 1)xb,2 0 (x > 0) I OSTAETSQ• U^ESTX P. 4.
72
1
iP=1
x41. nESKOLXKO WAVNYH NERAWENSTW |
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w SLEDU@]IH NIVE UTWERVDENIQH xi; yi |
2 C |
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PROIZWOLXNY. |
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2. iP=1 jxi + yijp |
iP=1 jxijp |
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+ iP=1 jyijp |
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(p 1). |
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3. eSLI RQDY |
i=1 jxij |
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SHODQTSQ (p |
+ q = 1;p; q > 1), TO RQD |
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PRI^EM |
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xiyi |
SHODITSQ ABSOL@TNO |
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j 1 xiyij " 1 jxijp#1=p " |
1 jyijq#1=q : |
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(p 1), TO SHODITSQ RQD |
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4. |
eSLI RQDY |
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i=1 jxi + yij |
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nERAWENSTWO P. 3 NAZYWAETSQ NERAWENSTWOM g•ELXDERA, P. 4 | mINKOWSKO- GO. eSLI, p = q = 2, TO NERAWENSTWA PP. 3,4 NAZYWA@TSQ SOOTWETSTWENNO NERAWENSTWAMI kO[I-bUNQKOWSKOGO I {WARCA.
pP. 3, 4 O^EWIDNYM OBRAZOM SLEDU@T IZ P. 1 I 2 SOOTWETSTWENNO. w |
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SWO@ O^EREDX, P. 2 QWLQETSQ SLEDSTWIEM P. 1: |
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i=1 jxi + yijp |
i=1 jxijjxi + yijp,1 |
+ i=1 jyijjxi + yijp,1 |
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i=1 jxijp i=1 jxi + yijp |
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(TAK KAK (p |
1)q = p), I OSTAETSQ• |
RAZDELITX OBE ^ASTI POLU^ENNOGO NE- |
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RAWENSTWA NA |
iP=1 jxi + yijp |
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73
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p. 1, KAK NETRUDNO WIDETX, SLEDUET IZ NERAWENSTWA |
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(1) |
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xip#1=p " n yiq#1=q |
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oSTALOSX DOKAZATX (1). wWEDEM• FUNKCI@ |
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f (x) = x , x + , 1 (x > 0) PRI 0 < < 1: |
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iMEEM |
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> 0 |
PRI 0 < x < 1, |
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f0(x) = (x ,1 , 1) < 0 |
PRI x > 1. |
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sLEDOWATELXNO, |
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(2) |
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PRI^EM• LEWAQ ^ASTX OBRA]AETSQ W NULX W EDINSTWENNOJ TO^KE x = 1 (ZDESX |
IMEET MESTO MAKSIMUM). pOLAGAQ W (2) = |
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(TAK ^TO 1, = |
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GDE a = xip |
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sUMMIRUQ \TI NERAWENSTWA PO i, POLU^AEM (1). >
74
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perwoobraznaq i neopredelennyj integral
x42. pERWOOBRAZNAQ I NEOPREDEL•ENNYJ INTEGRAL
zNAQ \LEMENTARNU@ FUNKCI@, MY UMEEM NAJTI EE• PROIZWODNU@. oB- RATNAQ ZADA^A | OTYSKANIE FUNKCII PO EE• PROIZWODNOJ. k E•E RE[ENI@ MY PEREHODIM.
1. pUSTX E( R) OTKRYTO. fUNKCIQ F : E ! R NAZYWAETSQ PERWO- OBRAZNOJ DLQ FUNKCII f : E ! R, ESLI F DIFFERENCIRUEMA I F 0(x) = f (x) (x 2 E). eSTESTWENNO SPROSITX, DLQ KAVDOJ LI FUNKCII f SU]ESTWU- ET PERWOOBRAZNAQ? oKAZYWAETSQ, NET, NE DLQ WSQKOJ. oDNAKO NIVE BUDET POKAZANO, ^TO \TO WERNO DLQ KAVDOJ NEPRERYWNOJ FUNKCII. w \TOM RAZ- DELE WSE FUNKCII PREDPOLAGA@TSQ NEPRERYWNYMI BEZ OSOBYH NA TO OGO- WOROK. s^ITAETSQ TAKVE, ^TO OBLASTX@ OPREDELENIQ WSEH WSTRE^A@]IHSQ FUNKCIJ QWLQETSQ NEKOTORYJ INTERWAL (a; b).
2. eSLI F | PERWOOBRAZNAQ DLQ f , TO L@BAQ DRUGAQ PERWOOBRAZNAQ G DLQ f WYRAVAETSQ FORMULOJ G = F +C, GDE C | NEKOTORAQ POSTOQNNAQ. |TO SLEDUET IZ 32.4. zDESX SU]ESTWENNO, ^TO f ZADANA NA INTERWALE!
3. nEOPREDEL•ENNYM INTEGRALOM OT NEPRERYWNOJ FUNKCII f NAZYWA-
ETSQ SOWOKUPNOSTX WSEH EE• PERWOOBRAZNYH. oBOZNA^ENIE: Z |
f(x)dx. tA- |
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KIM OBRAZOM, ESLI F | NEKOTORAQ PERWOOBRAZNAQ DLQ f , TO |
Z f (x)dx = |
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fF (x) + Cj C 2 Rg. bUDEM DALEE ISPOLXZOWATX BOLEE KOROTKU@ ZAPISX: |
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Z f (x)dx = F (x) + C . pONQTIE NEOPREDELENNOGO• |
INTEGRALA UDOBNO DLQ |
OWLADENIQ TEHNIKOJ OTYSKANIQ PERWOOBRAZNYH OT [IROKOGO KLASSA \LE- MENTARNYH FUNKCIJ.
x43. sWOJSTWA NEOPREDELENNOGO• INTEGRALA
pRIWED•EM NESKOLXKO SWOJSTW NEOPREDELENNOGO• INTEGRALA, POLEZNYH DLQ OTYSKANIQ PERWOOBRAZNYH.
1. Z (f(x) + g(x))dx = Z f(x)dx + Z g(x)dx,
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Z |
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f (x)dx = |
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f (x)dx (0 = |
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R). |
2. [fORMULA INTEGRIROWANIQ PO ^ASTQM]:
Z f (x)g0(x)dx = f (x)g(x) , Z f0 (x)g(x)dx:
oTMETIM, ^TO PRIWEDENNU@• FORMULU UDOBNO ISPOLXZOWATX W SLEDU@]EJ FORME: Z f (x)dg(x) = f(x)g(x) , Z g(x)df(x).
3. [fORMULA ZAMENY PEREMENNOJ]:
Z f(t)dt = Z f ('(x))'0(x)dx; t = '(x) (ZDESX SPRAWA I SLEWA STOQT FUNKCII OT x).
pP. 1,2 PROWERQ@TSQ DIFFERENCIROWANIEM. fORMULA P. 3 SLEDUET IZ |
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GDE dxZ f(t)dt | SEMEJSTWO PROIZWODNYH FUNKCIJ KLASSA Z f (t)dt (ONO |
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SWODITSQ K ODNOJ FUNKCII f ('(x))'0(x)). |
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dLQ OTYSKANIQ PERWOOBRAZNYH NA PRAKTIKE POLEZNA TABLICA, PROWER- |
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KA KOTOROJ PROIZWODITSQ DIFFERENCIROWANIEM (SM. NIVE). |
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sinx |
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d cosx |
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p R I M E R Y. 4. Z |
tg xdx = Z |
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cos xdx = ,Z |
cosx = , ln j cos xj + C. |
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5. Z |
ln xdx = x ln x , Z xd ln x = x ln x , Z |
dx = x(ln x , 1) + C . |
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x |
6. J |
=xZ ex cosxdx = ex sin x |
, Z ex sin xdx = ex sin x + |
Z exd cosx = |
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e |
sinx + e |
cosx , J + C. rE[AQ POLU^ENNOE URAWNENIE OTNOSITELXNO J, |
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NAHODIM J = 2e (sin x + cosx) + C. |
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7. Jm = Z (a2 + x2 )m (m 2 N). pRI m = 1 | \TO TABLI^NYJ INTEGRAL. |
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pRI m > 1 ISPOLXZUEM REKURRENTNU@ FORMULU |
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1 |
"Jm,1 , Z |
x2dx |
# |
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xd(a2 + x2) |
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Jm = |
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(a2 + x2)m |
= a2 Jm,1 |
, 2a2 Z |
(a2 + x2 )m |
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2a (m , 1)(a + x ) , |
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76
![](/html/2706/47/html_HnPYSFrBEL.Stq_/htmlconvd-Z_YwAJ77x1.jpg)
Z |
(x |
, |
a)ndx = |
(x , a)n+1 |
+ C (n = |
1); |
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n + 1 |
6 , |
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Zxdx, a = ln jx , aj + C;
Zaxdx = ln1aax + C;
Zexdx = ex + C;
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dx |
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x |
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Z p |
a2 |
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x2 |
= arcsin a + C (a > 0); |
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dx |
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= ln x + pa |
2 |
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Z pa2 + x2 |
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+ x + C ; |
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arctg |
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Z a + x |
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6 |
Zcosxdx = sin x + C;
Zsinxdx = , cos x + C;
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Z cos2 x |
= tg x + C ; |
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dx |
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Z sin2 x |
= , ctg x + C ; |
Zsindxx = ln j tg x2j + C;
Zsh xdx = chx + C;
Zchxdx = shx + C.
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dx |
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8. |
Z (x2 + px + q)m ( = p ,4q < 0; m 2 N). pODSTANOWKOJ t = x +p=2 |
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\TOT INTEGRAL SWODITSQ K P. 7. |
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9. |
J |
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= |
Z x2 + px |
+ q |
( = p , |
4q |
> 0). iMEEM x + px + q = |
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(x |
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1)(x |
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2 ); 1 |
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= 2, I |
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J = |
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1 |
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1 |
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dx = |
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1 |
ln |
x , 1 |
+ C: |
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1 , 2 Z x , 1 , x , 2 |
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( x + )dx |
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(2x + p)dx |
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2 |
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10. Z |
x2 + px + q = |
2 Z x2 + px + q |
+ 2 |
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p |
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= |
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ln |
x2 + px + q |
+ |
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dx |
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( = 0). |
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, 2 Z x2 + px + q |
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11. Z |
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( x + )dx |
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(m > 1; < 0). pRI•EMOM P. 10 SWODITSQ K P. 7. |
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(x2 + px + q)m |
77
![](/html/2706/47/html_HnPYSFrBEL.Stq_/htmlconvd-Z_YwAJ78x1.jpg)
12. z A M E ^ A N I E. sU]ESTWU@T \LEMENTARNYE FUNKCII (NAPRIMER,
e,x2 ; sinxx ), PERWOOBRAZNYE DLQ KOTORYH ^EREZ \LEMENTARNYE FUNKCII UVE NE WYRAVA@TSQ. dOKAZATELXSTWO \TOGO, ODNAKO, WESXMA NEPROSTO.
x44. oTYSKANIE PERWOOBRAZNYH DLQ RACIONALXNYH FUNKCIJ
1. pOZWOLIM SEBE WOLXNOSTX: FUNKCII P (x) I P (x)R(x), GDE P (x),
Q(x) Q(x)R(x)
Q(x), R(x) | NEKOTORYE POLINOMY, BUDEM S^ITATX RAWNYMI, HOTQ U NIH, WOOB]E GOWORQ, RAZNYE OBLASTI OPREDELENIQ. pUSTX OTNO[ENIE QP ((xx)) DWUH
POLINOMOW QWLQETSQ PRAWILXNOJ NESOKRATIMOJ DROBX@, PRI^EM• KO\FFI- CIENT PRI STAR[EJ STEPENI U POLINOMA Q(x) RAWEN 1, TAK ^TO
(1) Q(x) = (x |
, |
a) : : : |
(x |
, |
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b) (x2 |
+ px + q) : : : (x2 + rx + s) |
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(p2 , 4q < 0; : : : ; r2 |
, 4s < 0). |
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2. pRI SDELANNYH PREDPOLOVENIQH IMEET MESTO ODNOZNA^NO OPRE- |
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DEL•ENNOE PREDSTAWLENIE |
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P (x) |
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A1 |
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A2 |
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A |
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Q(x) = |
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(x , a) |
+ (x |
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, a) ,1 + : : : + x , a + : : : |
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B1 |
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B2 |
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+ |
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(x , b) + (x |
, b) ,1 + : : : + x , b |
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C x + D |
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C1x + D1 |
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C2x + D2 |
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+ |
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(x2 + px + q) + (x2 |
+ px + q) ,1 |
+ : : : + x2 + px + q |
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E1x + F1 |
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E x |
+ F |
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+ : : : + 2 |
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(x + rx + s) |
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pREDWARITELXNO USTANOWIM LEMMU:
3. pUSTX W OBOZNA^ENIQH (1) u(x) I v(x) | POLINOMY, ODNOZNA^NO OPREDEL•ENNYE RAWENSTWAMI
Q(x) = (x , a) u(x) = (x2 + px + q) v(x):
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P (x) |
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A1 |
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C1x + D1 |
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tOGDA Q(x) = |
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(x |
, |
a) + S1 (x) = (x2 |
+ px + q) + S2 |
(x), GDE S1 (x) = |
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R(x) |
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T(x) |
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PRAWILXNYE DROBI |
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(x , a) ,1u(x); S2(x) = (x2 |
+ px + q) ,1v(x) | |
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dLQ DOKAZATELXSTWA 1-GO RAWENSTWA POLOVIM |
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P (a) |
1 |
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(P (x) , A1u(x)); |
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A1 = |
u(a) ; R(x) = |
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x , a |
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78
![](/html/2706/47/html_HnPYSFrBEL.Stq_/htmlconvd-Z_YwAJ79x1.jpg)
GDE R(x) | NA SAMOM DELE POLINOM (TAK KAK a | KORENX POLINOMA
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PRI^EM |
P (x) |
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A1 |
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PRAWILXNAQ DROBX |
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P (x) , A1u(x)), |
Q(x) , (x , a) |
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• S1 (x) = |
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dLQ DOKAZATELXSTWA |
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GO RAWENSTWA WOZXMEM W KA^ESTWE |
C1 |
I |
D1 |
RE[ENIQ |
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2- |
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SISTEMY URAWNENIJ |
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8 C1 + D1 = |
P ( ) |
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(2) |
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v( ) |
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< C1 + D1 |
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KORNI TREH^LENA• |
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GDE I | KOMPLEKSNO SOPRQVENNYE• |
x |
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TEMA ODNOZNA^NO RAZRE[IMA, TAK KAK E•E DETERMINANT |
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pRI \TOM C1 I D1 WE]ESTWENNY (!!). pOLOVIM |
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1 |
[P (x) , (C1x + D1)v(x)]: |
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T (x) = |
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x2 + px + q |
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mOVNO UBEDITXSQ, ^TO T(x) | POLINOM I ^TO DROBX S2(x) PRAWILXNAQ (RASSUVDENIQ PRI \TOM ANALOGI^NY PRIWEDENNYM• WY[E). >
4. [dOKAZATELXSTWO P. 2]. sU]ESTWOWANIE RAZLOVENIQ SLEDUET IZ DO- KAZANNOJ LEMMY, POZWOLQ@]EJ POSLEDOWATELXNYM PONIVENIEM STEPENI POLINOMA Q(x) POLU^ITX ISKOMOE RAWENSTWO. eDINSTWENNOSTX SLEDUET IZ TOGO, ^TO KONSTANTY A1; : : : ; F OPREDELQ@TSQ ODNOZNA^NO:
A1 |
= lim (x |
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a) |
P (x) |
; A2 |
= lim(x |
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a) ,1 (P (x) |
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A1 |
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Q(x) |
, |
, (x , a) |
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x!a |
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x!a |
Q(x) |
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I T. D. tAKVE POSLEDOWATELXNO OPREDELQ@TSQ WELI^INY C1; : : : ; F . dEJST- WITELXNO, C1; D1 NEOBHODIMO UDOWLETWORQ@T SISTEME URAWNENIJ (2), KO- TORAQ, KAK OTME^ALOSX, RAZRE[IMA ODNOZNA^NO I T. D.
5. iTAK, ZADA^A OTYSKANIQ PERWOOBRAZNOJ DLQ RACIONALXNOJ FUNKCII
P (x) SWODITSQ K OTYSKANI@ KORNEJ POLINOMA Q(x). kOLX SKORO KORNI
Q(x)
NAJDENY, TO, ZAPISAW PREDSTAWLENIE P. 2, MOVNO POLU^ITX WYRAVENIE DLQ ISKOMOJ PERWOOBRAZNOJ ^EREZ \LEMENTARNYE FUNKCII (SM. PP. 43.8{12).
6. z A M E ^ A N I E. eSLI P (x) | NEPRAWILXNAQ DROBX, TO, POLXZUQSX,
Q(x)
NAPRIMER, ALGORITMOM eWKLIDA, SLEDUET PREDWARITELXNO PREOBRAZOWATX
79
EE• K WIDU P (x) = S(x)+R(x), GDE S(x) | POLINOM, A R(x) | PRAWILXNAQ
Q(x) Q(x) Q(x)
DROBX. pOSLE \TOGO OSTA•ETSQ WOSPOLXZOWATXSQ RAZLOVENIEM P. 2 DLQ DROBI
R(x). Q(x)
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x5 + 3x2 + 2 |
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7. p R I M E R. wY^ISLIM J = Z x4 + x3 |
+ x + 1dx. dROBX, STOQ]AQ |
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POD ZNAKOM INTEGRALA, NEPRAWILXNAQ. pREOBRAZUEM PODYNTEGRALXNOE WY- |
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RAVENIE SOGLASNO PP. 2,6: |
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x + 1 |
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dLQ NAHOVDENIQ NEIZWESTNYH KO\FFICIENTOW NA PRAKTIKE PRIWODQT DRO- BI K OB]EMU ZNAMENATEL@ I PRIRAWNIWA@T KO\FFICIENTY POLINOMOW W ^ISLITELQH PRI ODINAKOWYH STEPENQH x. pOSTUPAQ TAK, POLU^IM SISTEMU LINEJNYH URAWNENIJ. rE[AQ EE•, NAHODIM B = C = ,1; D = ,1=3; A = 4=3. tAKIM OBRAZOM,
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3x + 1 |
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J = |
(x + 2 + |
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3(x + 1)2 , x + 1 |
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x + 1))dx |
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x2 |
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= |
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+ 2x |
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3(x + 1) |
, lnjx + 1j , |
3Z |
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x2 , x + 1dx: |
pOSLEDNIJ INTEGRAL S^ITAETSQ SPOSOBOM PRIMERA 43.11.
80