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Çàóâàæåííÿ. Âèïàäîê α = 0 íå âèêëþ÷àºòüñÿ.  öüîìó<br />
âèïàäêó f(x) =P n (x), à y ÷.í. = Q n (x)x r , äå r — ÷èñëî êîðåí³â<br />
õàðàêòåðèñòè÷íîãî ð³âíÿííÿ, ÿê³ äîð³âíþþòü íóëþ.<br />
Ïðèêëàä 11.7.4 Çíàéòè çàãàëüíèé ðîçâ’ÿçîê ð³âíÿííÿ<br />
x<br />
y′′ − 5y′<br />
+ 6y = e . (11.7.5)<br />
Ðîçâ’ÿçàííÿ. Ñêëàäàºìî õàðàêòåðèñòè÷íå ð³âíÿííÿ ³<br />
2<br />
çíàõîäèìî éîãî êîðåí³: k − 5k+ 6 = 0, k1 = 2, k2<br />
= 3. Çàãàëüíèé<br />
ðîçâ’ÿçîê îäíîð³äíîãî ð³âíÿííÿ y ç.î. ìຠâèãëÿä:<br />
2x<br />
3x<br />
y ç.î. ce<br />
1<br />
ce<br />
2<br />
= + .<br />
Îñê³ëüêè k 1 =2 ³ k 2 = 3 íå ð³âí³ α = 1, òî ÷àñòèííèé ðîçâ’ÿçîê<br />
íåîäíîð³äíîãî ð³âíÿííÿ y ÷.í. ñë³ä øóêàòè ó âèãëÿä³<br />
x<br />
y ÷.í. = Ae ,<br />
äå À – íåâ³äîìà ñòàëà.<br />
x<br />
x<br />
Çíàõîäèìî y′ ÷.í. ³ y′′ ÷.í. :y′ ÷.í.<br />
= Ae y′′ ÷.í.<br />
= Ae . ϳäñòàâëÿþ÷è<br />
âèðàçè y ÷.í. , y′′ ÷.í. , ³ y′′′ ÷.í. äî ð³âíÿííÿ (11.7.5) ³ ñêîðî÷óþ÷è íà<br />
x<br />
ìíîæíèê e ≠ 0 îäåðæóºìî ð³âí³ñòü: A –5A +6A = 1, çâ³äê³ëÿ<br />
A = 1/2.<br />
Îòæå, ÷àñòèííèé ðîçâ’ÿçîê ð³âíÿííÿ 11.7.5 ìຠâèãëÿä:<br />
1 x<br />
y ÷.í.<br />
= e , à çàãàëüíèé ðîçâ’ÿçîê éîãî òàêèé:<br />
2<br />
2<br />
y<br />
x 3x 1 x<br />
ç.í. = ce<br />
1<br />
+ ce<br />
2<br />
+ e .<br />
2<br />
Ïðèêëàä 11.7.5. Ðîçâ’ÿçàòè ð³âíÿííÿ<br />
y′′ − 7y′<br />
+ 6 y = ( x − 2) e<br />
x . (11.7.6)<br />
Ð î ç â ’ ÿ ç à í í ÿ. Òóò ïðàâà ÷àñòèíà ìຠâèãëÿä:<br />
( ) 1 x<br />
P1 x = e ⋅ , ïðè÷îìó êîåô³ö³ºíò 1 â ïîêàçíèêó ñòåïåí³ º ïðîñòèì<br />
êîðåíåì õàðàêòåðèñòè÷íîãî ìíîãî÷ëåíà k 2 –7k +6=0,<br />
k 1 = 1, k 2 = 6. Îòæå, ÷àñòèííèé ðîçâ’ÿçîê øóêàºìî ó âèãëÿä³<br />
y ÷.í. =(Ax + B)xe x . ϳäñòàâëÿºìî éîãî äî ð³âíÿííÿ (11.7.6),<br />
ìàºìî:<br />
[(Ax 2 + Bx) +(4Ax +2B) +2A –7(Ax 2 + Bx) – 7(2Ax +B) +<br />
+6(Ax 2 + Bx)]e x =(x –2)e x . (11.7.7)<br />
Ïðè öüîìó ìè âèêîðèñòîâóâàëè, ùî<br />
y′ ÷.í. =(2Ax +B)e x +(Ax 2 +Bx)e x =(Ax 2 +(B +2A)x + B)e x<br />
y′′ ÷.í. =(2Ax +B +2A)e x +(Ax 2 +(B +2A)x + B)e x =<br />
=[Ax 2 +(B +4A)x +2B +2A]e x .<br />
Ñêîðî÷óþ÷è â (11.7.7) íà e x ≠ 0 ³ ïðèð³âíþþ÷è êîåô³ö³ºíòè<br />
ïðè ð³âíèõ ñòåïåíÿõ x, îòðèìàºìî: –10A =1,<br />
1 9<br />
–5B +2A = –2, çâ³äêè A =− , B = . Îòæå, ÷àñòèííèì<br />
10 25<br />
ðîçâ’ÿçêîì ð³âíÿííÿ 11.7.6 º<br />
⎛ 1 9 ⎞<br />
y ÷.í.<br />
= x⎜− x + e<br />
10 25<br />
⎟<br />
⎝<br />
⎠<br />
à éîãî çàãàëüíèì — ôóíêö³ÿ:<br />
⎛ 1 9 ⎞<br />
⎜<br />
10 25<br />
⎟<br />
⎝<br />
⎠<br />
Ïðèêëàä 11.7.6. Ðîçâ’ÿçàòè ð³âíÿííÿ<br />
x 6x x<br />
y ç.í. = ce<br />
1<br />
+ ce<br />
2<br />
+ x − x+<br />
e<br />
x<br />
x<br />
y′′ − 2y′<br />
+ y = e . (11.7.8)<br />
Ðîçâ’ÿçàííÿ. Ñêëàäåìî õàðàêòåðèñòè÷íå ð³âíÿííÿ<br />
k 2 –2k + 1 = 0. Éîãî êîðåí³ k 1 = k 2 = 1. Çàãàëüíèé ðîçâ’ÿçîê<br />
îäíîð³äíîãî ð³âíÿííÿ ìຠâèãëÿä:<br />
y ç.o. = e x (c 1 + xc 2 ).<br />
×àñòèííèé ðîçâ’ÿçîê íåîäíîð³äíîãî ð³âíÿííÿ ó â³äïîâ³äíîñò³<br />
äî ôîðìóëè (11.7.4) (k 1 = k 2 = α = 1) ñë³ä øóêàòè ó<br />
âèãëÿä³: y ç.í. =Ax 2 e x . Çíàõîäèìî ïåðøó ³ äðóãó ïîõ³äí³ ö³º¿<br />
ôóíêö³¿:<br />
,<br />
.<br />
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