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4 3<br />
∆Ψ = 10 − 10 = 9000 .<br />
Îòæå, ïðè ðåàë³çàö³¿ ïðîäóêòó â 100 îäèíèöü äîõîä ñòàíîâèòü<br />
11 000 (ãð. îä), à ïðèáóòîê 9000 (ãð. îä.).<br />
9.12. ÍÀÁËÈÆÅÍÅ ÎÁ×ÈÑËÅÍÍß<br />
ÂÈÇÍÀ×ÅÍÈÕ ²ÍÒÅÃÐÀ˲Â<br />
Äëÿ íàáëèæåíîãî îá÷èñëåííÿ âèçíà÷åíèõ ³íòåãðàë³â º<br />
ê³ëüêà ñïîñîá³â. ßêùî ôóíêö³ÿ f(x) çàäàíà ôîðìóëîþ àáî<br />
òàáëèöåþ, òî íàáëèæåíå çíà÷åííÿ âèçíà÷åíîãî ³íòåãðàëà<br />
b<br />
∫ fxdx ( ) ìîæíà çíàéòè òàêèì øëÿõîì: 1) ðîçáèòè â³äð³çîê<br />
a<br />
³íòåãðóâàííÿ [a, b] òî÷êàìè x 1 , x 2 , ..., x n-1 íà n ð³âíèõ ÷àñòèí<br />
h = ; 2) îá÷èñëèòè çíà÷åííÿ ï³ä³íòåãðàëüíî¿ ôóíê-<br />
b − a<br />
n<br />
ö³¿ y = f(x) â òî÷êàõ a, x 1 , x 2 , ..., x n-1 , b, òîáòî âèçíà÷èòè:<br />
y 0 = f(a), y 1 = f(x 1 ), ..., y n-1 = f(x n-1 ), y n = f(b); 3) ñêîðèñòàòèñÿ<br />
îäí³ºþ ç íàáëèæåíèõ ôîðìóë.<br />
Íàéá³ëüøå âæèâàþòüñÿ òàê³ íàáëèæåí³ ôîðìóëè, ÿê³ çàñíîâàí³<br />
íà ãåîìåòðè÷íîìó çîáðàæåíí³ âèçíà÷åíîãî ³íòåãðàëà<br />
ó âèãëÿä³ ïëîù³ êðèâîë³í³éíî¿ òðàïåö³¿. Ïðè öüîìó ïðèïóñêàºòüñÿ,<br />
ùî ôóíêö³ÿ f(x) çàäàíà àíàë³òè÷íî ³ äèôåðåíö³éîâíà<br />
ïîòð³áíå ÷èñëî ðàç äëÿ îö³íêè ïîõèáêè.<br />
9.12.1. Ôîðìóëà ïðÿìîêóòíèê³â<br />
àáî<br />
b<br />
n−1<br />
∫ fxdx ( ) ≈ hy ( 0<br />
+ y1 + K + yn−<br />
1)<br />
= h∑<br />
yi<br />
(9.12.1)<br />
a<br />
i=<br />
0<br />
b<br />
n<br />
∫ fxdx ( ) ≈ hy ( 1<br />
+ y2<br />
+ K + yn)<br />
= h∑<br />
yi<br />
. (9.12.2)<br />
a<br />
i=<br />
1<br />
Ãåîìåòðè÷íî (ðèñ. 9.29) çà ö³ºþ ôîðìóëîþ ïëîùà êðèâîë³í³éíî¿<br />
òðàïåö³¿ aABb, ÿêà â³äïîâ³äຠ³íòåãðàëó ∫ fxdx ( ) ,<br />
b<br />
a<br />
çàì³íþºòüñÿ ñóìîþ ïëîù çàøòðèõîâàíèõ ïðÿìîêóòíèê³â.<br />
Ðèñ. 9.29<br />
9.12.2. Ôîðìóëà òðàïåö³é<br />
b<br />
⎛y 1<br />
0<br />
+ y n<br />
n<br />
⎞ ⎛y0<br />
+ y −<br />
n ⎞<br />
∫ fxdx ( ) ≈ h⎜ + y1 + y2 + K + yn−1⎟ = h⎜ + ∑ yi⎟. (9.12.3)<br />
a<br />
⎝ 2 ⎠ ⎝ 2 i=<br />
1 ⎠<br />
Ãåîìåòðè÷íî çà ö³ºþ ôîðìóëîþ ïëîùà êðèâîë³í³éíî¿<br />
òðàïåö³¿ çàì³íþºòüñÿ ñóìîþ ïëîù çàøòðèõîâàíèõ òðàïåö³é<br />
(ðèñ. 9.30).<br />
Ðèñ. 9.30<br />
1<br />
dx<br />
Ïðèêëàä 9.1.7. Îá÷èñëèòè íàáëèæåíî ³íòåãðàë ∫ 2 çà<br />
0 1 + x<br />
äîïîìîãîþ ôîðìóë ïðÿìîêóòíèê³â ³ òðàïåö³¿ é ïîð³âíÿòè ç<br />
éîãî òî÷íèì çíà÷åííÿì.<br />
Ðîçâ’ÿçàííÿ. Íå îáìåæóþ÷è çàãàëüíîñò³, ðîç³á’ºìî<br />
ñåãìåíò [0, 1] íà 10 ð³âíèõ ÷àñòèí (h =10 –1 ) ³ çíàéäåìî çíà-<br />
÷åííÿ ôóíêö³¿ f(x) =(1+x 2 ) –1 â òî÷êàõ ðîçáèòòÿ. Ïîò³ì çàñòîñóºìî<br />
ôîðìóëè (9.10.1) – (9.10.3)<br />
1<br />
∫<br />
0<br />
dx<br />
1 +<br />
(1.0 0.99 0.96 0.92 0.86 0.8<br />
2<br />
x ≈ + + + + + +<br />
340 341