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Ïðèêëàä 11.6.2. Ïîêàçàòè, ùî ôóíêö³¿ ( )<br />
( )<br />
1<br />
kx<br />
= ,<br />
y x e<br />
y2 x = 5e<br />
kx (k ∈ R ) ë³í³éíî çàëåæí³ íà (–∞, ∞), òîáòî ë³í³éíî<br />
çàëåæí³ ïðè x ∈ R .<br />
Ä î â å ä å í í ÿ. Ñêëàäåìî ë³í³éíó êîìá³íàö³þ<br />
( )<br />
α e + α 5e = e α + 5α .<br />
kx kx kx<br />
1 2 1 2<br />
ßêùî òåïåð ó ö³é ð³âíîñò³ ïîêëàäåìî α 1 =–5α 2 , α 2 ≠ 0, òî<br />
∀x∈R áóäåìî ìàòè òîòîæí³ñòü<br />
kx<br />
α e + 5α e ≡ 0,<br />
kx<br />
1 2<br />
ïðè÷îìó α 1 ³ α 2 íå äîð³âíþþòü íóëþ.<br />
Çã³äíî ç îçíà÷åííÿì 11.6.1 ôóíêö³¿<br />
1 ( )<br />
( ) = ë³í³éíî çàëåæí³ ïðè x∈R.<br />
Çàóâàæåííÿ 1. Ó ïðèêëàä³ 11.6.2 y2( x) 5y1( x)<br />
y x e<br />
2<br />
5 kx<br />
y x e<br />
kx<br />
= òà<br />
= . Ïðè<br />
öüîìó âèÿâèëîñÿ, ùî âîíè ë³í³éíî çàëåæí³. Öåé ïðèêëàä<br />
óçàãàëüíþºòüñÿ. Ìຠì³ñöå òàêå òâåðäæåííÿ: äëÿ òîãî ùîá<br />
ôóíêö³¿ y 1 (x) òà y 2 (x) áóëè ë³í³éíî çàëåæíèìè, íåîáõ³äíî ³<br />
äîñòàòíüî, ùîá âîíè áóëè ïðîïîðö³éíèìè, òîáòî áóëè ïîâ’ÿçàí³<br />
òîòîæí³ñòþ<br />
( ) ≡λ ( ) àáî y ( x) y ( x)<br />
y x y x<br />
2 1<br />
1 2<br />
≡λ , x∈(a, b). (11.6.10)<br />
Í å î á õ³ ä í ³ ñ ò ü. Ôóíêö³¿ y 1 (x) òà y 2 (x) ë³í³éíî çàëåæí³,<br />
öå îçíà÷àº, ùî ³ñíóþòü òàê³ ÷èñëà α 1 ³ α 2 , ç ÿêèõ õî÷à á<br />
îäíå â³äì³ííå â³ä íóëÿ, òà ìຠì³ñöå òîòîæí³ñòü (11.6.7).<br />
Äëÿ êîíêðåòíîñò³ áóäåìî ââàæàòè, ùî öå ÷èñëî α 2 . Òîä³ ³ç<br />
òîòîæíîñò³ (11.6.7) âèïëèâຠy ( ) 1<br />
2<br />
x y1( x)<br />
α<br />
α<br />
≡− α<br />
2<br />
. Ïîçíà÷èìî<br />
1<br />
λ=− α<br />
. Òîä³ y 2 (x) =λy 1 (x) ³ ä³éñíî ìຠì³ñöå (11.6.10).<br />
2<br />
Äîñòàòí³ñòü. Íåõàé ôóíêö³¿ y 1 (x) òà y 2 (x) ïîâ’ÿçàí³<br />
òîòîæí³ñòþ (11.6.10). Íàïðèêëàä, ïåðøîþ, òîä³ òîòîæí³ñòü<br />
ìîæíà çàïèñàòè ó âèãëÿä³:<br />
( ) ( )<br />
1⋅y x −λy x ≡ 0. (11.6.11)<br />
2 1<br />
гâí³ñòü (11.6.11) îçíà÷àº, ùî y 1 (x) òà y 2 (x) ë³í³éíî çàëåæí³.<br />
Öå ä³éñíî òàê, òîìó ùî ³ç äâîõ ÷èñåë 1 ³ λ ñàìå 1<br />
â³äì³ííå â³ä íóëÿ.<br />
Àíàëîã³÷íî äîâîäèòüñÿ òâåðäæåííÿ ïðè α 1 ≠ 0.<br />
Îòæå, òâåðäæåííÿ äîâåäåíî. Íàäàë³, íå îáìåæóþ÷è çàãàëüíîñò³,<br />
áóäåìî ïðèïóñêàòè ïðè íàÿâíîñò³ çàëåæíîñò³ ôóíêö³¿<br />
y 1 (x) òà y 2 (x) ïåðøèé çàïèñ (11.6.10).<br />
Çàóâàæåííÿ 2. Î÷åâèäíî, ùî ÿêùî ôóíêö³¿ y 1 (x) òà<br />
y1<br />
( x)<br />
const<br />
y2<br />
( x)<br />
≠ .<br />
Ïðèêëàä 11.6.3. Äîâåñòè, ùî ôóíêö³¿ ( )<br />
y 2 (x) ë³í³éíî íåçàëåæí³, òî â³äíîøåííÿ<br />
2<br />
( )<br />
y x xe<br />
kx<br />
= , k∈R ë³í³éíî íåçàëåæí³ ïðè x∈R.<br />
Äîâåäåííÿ. Ðîçãëÿíåìî â³äíîøåííÿ<br />
1<br />
kx<br />
= ,<br />
y x e<br />
y2<br />
( x)<br />
y2<br />
( x)<br />
y ( x )<br />
( )<br />
1<br />
x<br />
y x = ,<br />
x∈R. Îñê³ëüêè x çì³ííà, òî çã³äíî ç çàóâàæåííÿì 2 ôóíêö³¿<br />
y 1 (x) òà y 2 (x) ë³í³éíî íåçàëåæí³. Ùî ³ òðåáà áóëî äîâåñòè.<br />
Çà îçíà÷åííÿì ç’ÿñóâàòè ë³í³éíó çàëåæí³ñòü àáî ë³í³éíó<br />
íåçàëåæí³ñòü ÷àñòî áóâຠâàæêî. Ó çâ’ÿçêó ç öèì âèíèêàþòü<br />
ïðîáëåìè. Âèð³øåííþ ¿õ äîïîìàãຠâèçíà÷íèê Âðîíñüêîãî<br />
1 y1 y2<br />
= yy′ 1 2<br />
− yy′<br />
2 1<br />
y′ y′<br />
. (11.6.12)<br />
1 2<br />
Âèçíà÷íèê Âðîíñüêîãî (âðîíñê³àí) ÿâëÿº ñîáîþ ôóíêö³þ,<br />
âèçíà÷åíó íà (a, b), ³ ïîçíà÷àºòüñÿ W(y 1 , y 2 ) àáî ïðîñòî W(x).<br />
Òåîðåìà 11.6.2. ßêùî ôóíêö³¿ y 1 (x) ³ y 2 (x) ë³í³éíî<br />
çàëåæí³ íà (a, b), òî âèçíà÷íèê Âðîíñüêîãî, ñêëàäåíèé ç íèõ,<br />
äîð³âíþº íóëþ íà öüîìó ³íòåðâàë³.<br />
Äîâåäåííÿ. Îñê³ëüêè ôóíêö³¿ y 1 (x) ³ y 2 (x) ë³í³éíî çàëåæí³,<br />
òî ó â³äïîâ³äíîñò³ äî çàóâàæåííÿ 1 âîíè ïîâ’ÿçàí³<br />
ì³æ ñîáîþ ð³âí³ñòþ:<br />
1<br />
Âðîíñüêèé Þçåô (1775 – 1853) — ïîëüñüêèé ìàòåìàòèê.<br />
1<br />
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