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Öÿ ôóíêö³ÿ ÿâëÿº ñîáîþ äîáóòîê äâîõ ôóíêö³é: y = x òà<br />
1<br />
y = sin . Ïåðøà ôóíêö³ÿ â òî÷ö³ õ = 0 âèçíà÷åíà, à äðóãà<br />
x<br />
ôóíêö³ÿ â òî÷ö³ õ = 0 íå âèçíà÷åíà. Òîìó ³ ñàìà ôóíêö³ÿ<br />
1<br />
y= x⋅ sin â òî÷ö³ õ = 0 íå âèçíà÷åíà. Òåïåð â³çüìåìî áóäüÿêó<br />
íåñê³í÷åííî ìàëó ïîñë³äîâí³ñòü {x n }.<br />
x<br />
1<br />
³äïîâ³äí³ çíà÷åííÿ yn<br />
= xn<br />
⋅ sin òåæ óòâîðþþòü íåñê³íxn<br />
÷åííî ìàëó ïîñë³äîâí³ñòü (ïîñë³äîâí³ñòü {y n } — íåñê³í÷åííî<br />
ìàëà ÿê äîáóòîê íåñê³í÷åííî ìàëî¿ ïîñë³äîâíîñò³ {x n } òà<br />
⎛ 1 ⎞<br />
îáìåæåíî¿ ïîñë³äîâíîñò³<br />
⎜<br />
sin ≤ 1<br />
x ⎟). Òàêèì ÷èíîì, ïîâåä³íêà<br />
ôóíêö³¿ ïðè ïðÿìóâàíí³ õ äî íóëÿ ö³ëêîì âèçíà÷åíà. ¯¿<br />
⎝ n ⎠<br />
çíà÷åííÿ òåæ ïðÿìóþòü äî íóëÿ (àáî ïîñë³äîâí³ñòü {y n }<br />
çá³ãàºòüñÿ äî íóëÿ).<br />
Çðîáèìî òåïåð âèñíîâêè. Â ïðèêëàäàõ 6.2.1, 6.2.2, 6.2.5<br />
ïðè äèñêðåòíîìó ïðÿìóâàíí³ õ äî íóëÿ (âçàãàë³<br />
xn<br />
≠ 0, ∀n∈ N ) çíà÷åííÿ ôóíêö³¿ y n = f(x n ) óòâîðþþòü ïîñë³äîâí³ñòü<br />
{y n }, åëåìåíòè ÿêî¿ ïðÿìóþòü äî ö³ëêîì âèçíà÷åíîãî<br />
÷èñëà (â³äïîâ³äíî 0, 1, 0). Ö³ ÷èñëà õàðàêòåðèçóþòü ïîâåä³íêó<br />
â³äïîâ³äíèõ ôóíêö³é ïðè äèñêðåòíîìó ïðÿìóâàíí³ õ<br />
äî íóëÿ.<br />
Çàóâàæåííÿ. Ïðèêëàäè, àíàëîã³÷í³ ïðèêëàäàì 6.2.1,<br />
6.2.2, 6.2.5, ìîæíà íàâåñòè ïðè äîñë³äæåíí³ äåÿêî¿ ôóíêö³¿<br />
â áóäü-ÿê³é òî÷ö³ ä³éñíî¿ â³ñ³.  çâ’ÿçêó ç öèì ó ìàòåìàòèö³<br />
âèíèêëî âàæëèâå ïîíÿòòÿ: ãðàíèöÿ ôóíêö³¿ â òî÷ö³.<br />
Îçíà÷åííÿ 6.2.1. Íåõàé ôóíêö³ÿ y = f(x) âèçíà÷åíà íà<br />
ìíîæèí³ X = ( a, x0) ∪ ( x0,<br />
b)<br />
. Òîä³ ñòàëå ÷èñëî À íàçèâàºòüñÿ<br />
ãðàíèöåþ ôóíêö³¿ â òî÷ö³ õ = õ 0 (àáî êîëè õ → õ 0 ) ³ ñèìâîë³÷íî<br />
çàïèñóþòü<br />
lim fx ( ) = A,<br />
x→x0<br />
ÿêùî áóäü-ÿêà ïîñë³äîâí³ñòü {x n }, ÿêà íàëåæèòü ìíîæèí³ Õ<br />
xn<br />
x0,<br />
n N , íàðîäæóº ïîñë³äîâí³ñòü<br />
çíà÷åíü ôóíêö³¿ {f(x n )}, ùî çá³ãàºòüñÿ äî ÷èñëà<br />
À.<br />
Çã³äíî ç îçíà÷åííÿì 6.2.1 ãðàíèö³ ôóíêö³é, ðîçãëÿíóòèõ<br />
â ïðèêëàäàõ 6.2.1, 6.2.2, 6.2.5, áóäóòü â³äïîâ³äíî äîð³âíþâàòè<br />
0, 1, 0.<br />
Äîñë³äæåííÿ ïîâåä³íêè ôóíêö³¿ y = f(x) íà ìíîæèí³<br />
³ çá³ãàºòüñÿ äî òî÷êè õ = õ 0 ( ≠ ∀ ∈ )<br />
( , ) ( , )<br />
X = a x0 ∪ x0<br />
b , êîëè õ → õ 0 , ìîæíà çä³éñíèòè ³ ³íøèì<br />
øëÿõîì, äå çíà÷åííÿ àðãóìåíòó õ ðîçãëÿäàþòü íåäèñêðåòíî,<br />
à íåïåðåðâíî. Ïîÿñíèìî öå íà ïðèêëàä³ 6.2.1. Çà îçíà÷åííÿì<br />
6.2.1 limy<br />
= 0 . Öåé ôàêò ìîæíà ³íòåðïðåòóâàòè òàê:<br />
x→0<br />
ÿêèì áè íå áóëî ìàëèì ÷èñëî ε, çíàéäóòüñÿ òàê³ çíà÷åííÿ<br />
àðãóìåíòó õ, ùî çíà÷åííÿ ôóíêö³¿ áóäóòü ìåíøèìè öüîãî<br />
÷èñëà ε. Ðîçãëÿíåìî ìíîæèíó òî÷îê õ, ÿêà çàäîâîëüíÿº íåð³âí³ñòü<br />
x 0 (ðîçãëÿíóòà ìíîæèíà ïîâèííà áóòè ï³äìíîæèíîþ<br />
ìíîæèíè Õ). Òîä³ î÷åâèäíî, ùî ìíîæèíà çíà-<br />
÷åíü ôóíêö³¿ ó = õ çàäîâîëüíÿº íåð³âí³ñòü y 0 ∃δ( ε ) > 0 òàêå,<br />
ùî ∀ õ, ÿê³ çàäîâîëüíÿþòü âèìîãó<br />
0 < |x – õ 0 | < δ(ε),<br />
ôóíêö³ÿ y = f(x) çàäîâîëüíÿº íåð³âí³ñòü<br />
|f(x) – A| < ε.<br />
Öåé ôàêò çàïèñóþòü ó âèãëÿä³<br />
lim fx ( ) = A. (6.2.1)<br />
x→x0<br />
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