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Infinite Series & Definite Integral

๐Ÿ™…โ€โ™‚๏ธํœด๋Œ€ํฐ์œผ๋กœ ๋ณผ ๋•Œ ํ˜น์‹œ ๊ธ€์ž๋‚˜ ์ˆซ์ž๊ฐ€ ํ™”๋ฉด์— ๋‹ค ์•ˆ๋‚˜์˜ค๋ฉด, ํœด๋Œ€ํฐ ๊ฐ€๋กœ๋กœ ๋Œ๋ฆฌ์‹œ๋ฉด ๋ฉ๋‹ˆ๋‹ค

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<๋ชฉ์ฐจ>

1. ๋“ค์–ด๊ฐ€๋ฉฐ
2. ์ •์ ๋ถ„
3. ๋ฌดํ•œ๊ธ‰์ˆ˜์™€ ์ •์ ๋ถ„์˜ ๊ด€๊ณ„

1. ๋“ค์–ด๊ฐ€๋ฉฐ

๋ฌดํ•œ๊ธ‰์ˆ˜์™€ ์ •์ ๋ถ„์€ ๋—„๋ ˆ์•ผ ๋—„ ์ˆ˜ ์—†๋Š” ๊ด€๊ณ„๋ฅผ ๊ฐ–๊ณ  ์žˆ๋‹ค ๋ฌด์Šจ ๋ง์ด๋ƒ๋ฉด
๋ฌดํ•œ๊ธ‰์ˆ˜๋ฅผ ์ •์ ๋ถ„์œผ๋กœ ๋ฐ”๊พธ์–ด ๊ณ„์‚ฐํ•  ์ˆ˜ ์žˆ๊ณ  ์ •์ ๋ถ„์„ ๋ฌดํ•œ๊ธ‰์ˆ˜๋กœ ๋ฐ”๊พธ์–ด ๊ณ„์‚ฐํ•  ์ˆ˜ ์žˆ๋‹ค.

์ฐธ!๐Ÿค” ์ •์ ๋ถ„์„ ์–ธ์ œ ์“ฐ์ง€?
๊ตฌ๊ฐ„ [a, b]๊ฐ€ ์—ฐ์†์ธ ์–ด๋–ค ํ•จ์ˆ˜ f(x)๊ฐ€ ์žˆ๊ณ 
๊ทธ ๊ตฌ๊ฐ„ ์‚ฌ์ด์— ๊ฐ€๋กœ์˜ ๊ธธ์ด๊ฐ€ ๊ฐ™์€ n๊ฐœ์˜ ์ง๊ฐ์‚ฌ๊ฐํ˜•์˜ ๋„“์ด์˜ ํ•ฉ์˜ ๊ทนํ•œ๊ฐ’์„ ๊ตฌํ•  ๋•Œ

๐Ÿ”’PreRequisites

  1. limit
  2. sequence

*์ฐธ๊ณ 
๋ถ€์ •์ ๋ถ„์€ ๋ฌด์กฐ๊ฑด ์ƒ์ˆ˜๋ฅผ ๋ถ™์—ฌ ์ ๋ถ„ํ•ด์•ผํ•จ
$\int x= x^2+C$




2. ์ •์ ๋ถ„

๊ฐœ๋…์„ ์ •์˜ํ•˜๊ธฐ ์ „์— ์ •์ ๋ถ„์€ ์‰ฝ๊ฒŒ ๋งํ•ด ์ˆ˜์—ด์˜ ํ•ฉ์˜ ๊ทนํ•œ ๊ฐ’์œผ๋กœ
n๊ฐœ์˜ ์ง์‚ฌ๊ฐํ˜•๋“ค์˜ ๊ฐ ๋„“์ด์˜ ํ•ฉ์˜ ๊ทนํ•œ๊ฐ’์ด๋ผ๊ณ  ๋ณด๋ฉด ๋œ๋‹ค
์ž ๊น ์•„๋ž˜ ๊ทธ๋ฆผ์„ ๋ณด์ž


Desktop View

์ด ๊ทธ๋ฆผ์„ ๋Œ€์ถฉ ํ•œ๋ฒˆ ํ•œ๋ฒˆ ํ›‘์–ด ๋ณด๋‹ˆ 2๊ฐœ์˜ ์ฐจ์ด๋Š” ๊ทนํ•œ๊ฐ’ ์œ ๋ฌด๋‹ค
๊ทธ๋ƒฅ โ€œ์ด๋Ÿฐ๊ฒŒ ์žˆ๊ตฌ๋‚˜โ€์ •๋„๋กœ ์ƒ๊ฐํ•˜๊ณ  ๋„˜์–ด๊ฐ€๋ฉด ๋  ๊ฒƒ ๊ฐ™๋‹ค.
๋ฐ‘์—์„œ ์ œ๋Œ€๋กœ ์„ค๋ช…ํ•˜๊ฒ ๋‹ค


๊ฐœ๋…:

ํ•จ์ˆ˜ \(f(x)\)๊ฐ€ ๊ตฌ๊ฐ„ [a, b]์—์„œ ์—ฐ์†์ผ ๋•Œ, ๊ทธ ๊ตฌ๊ฐ„์„ n๋“ฑ๋ถ„ํ•˜์—ฌ ์–‘์ชฝ ๋๊ณผ ๊ฐ ๋ถ„์ ์˜ \(x\)์ขŒํ‘œ๋ฅผ \(x_0(=a), x_1, \cdot\cdot\cdot, x_{n-1}, x_n(=b)\)๋ผ ํ•˜๊ณ  \(\frac{b-a}{n} = \Delta x\)๋ผ ํ•  ๋•Œ
\(\lim_{n\to\infty}\sum\limits_{k=1}^Nf(x_k)\Delta x\) ์˜ ๊ฐ’์„ ํ•จ์ˆ˜ \(f(x)\)์˜ a์—์„œ b๊นŒ์ง€์˜ ์ •์ ๋ถ„์ด๋ผํ•˜๊ณ 
๊ทธ ๊ฐ’์„ \(\int^b_a f(x)dx\)๋กœ ๋‚˜ํƒ€๋‚ธ๋‹ค

\(\color{red}{\therefore}\) ย  ์ฆ‰ \(\lim_{n \to \infty} \sum\limits_{k=1}^N f(x_k) \Delta x\) \(\color{red}{=}\) \(\int^b_a f(x)dx\) ์ด๋‹ค

์‰ฌ์šด ์ดํ•ด๋ฅผ ์œ„ํ•ด ์•„๋ž˜ ๊ทธ๋ฆผ์„ ๋ณด์ž
(์•„๋ž˜ ๊ทธ๋ฆผ์€ \(\lim_{n \to \infty}\)๋ฅผ ๋บ€ n๊ฐœ์˜ ๋ชจ๋“  ๋ณด๋ผ ์ง์‚ฌ๊ฐํ˜•์˜ ๋„“์ด๋ฅผ \(\sum\limits\) ํ•œ ๊ฐ’์ด๋‹ค)

Desktop View

์—ฌ๊ธฐ์„œ ๋ณด๋ผ์ƒ‰ ๋ฒฝ๋Œ 1๊ฐœ ์นธ์˜ ๊ธธ์ด๋Š” \(\frac{b-a}{n}\)์ด๋‹ค
์™œ๋ƒํ•˜๋ฉด ์ „์ฒด ๊ธธ์ด๋Š” b-a๊ณ  n๋“ฑ๋ถ„ ํ–ˆ์œผ๋‹ˆ๊นŒ
์•„๋ฌดํŠผ ์ด๊ฒƒ์„ \(\Delta x\)๋กœ ๋‘์—ˆ๋‹ค (์ฆ‰ ๋ณด๋ผ์ƒ‰ ๋ฒฝ๋Œ ๊ฐ๊ฐ์˜ ๋ฐ‘๋ณ€์˜ ๊ธธ์ด๊ฐ€ ๋จ)

์ž์„ธํ•œ ์„ค๋ช…

์ง์‚ฌ๊ฐํ˜• ๋„“์ด์˜ ํ•ฉ

\(\color{red}{\frac{b-a}{n}}\) \(\left[ f\left(a+ \frac{b-a}{n}\right) + f\left(a+ \frac{b-a}{n}2\right) + \cdots f\left(a+ \frac{b-a}{n}n\right) \right]\)
์ด ์‹์€ \(\color{red}{๊ฐ€๋กœ}\)[1๋ฒˆ ์ง์‚ฌ๊ฐํ˜• ๋†’์ด + 2๋ฒˆ ์ง์‚ฌ๊ฐํ˜• ๋†’์ด + โ€ฆ + ๋งˆ์ง€๋ง‰ ์ง์‚ฌ๊ฐํ˜• ๋†’์ด ]์ด๋‹ค
์—ฌ๊ธฐ์— f(ํ•จ์ˆซ๊ฐ’)์„ ๊ทธ๋ž˜ํ”„์— ๋„ฃ์œผ๋ฉด ๋†’์ด๊ฐ€ ์ž์—ฐ์Šค๋Ÿฝ๊ฒŒ ๋‚˜์˜ฌ ๊ฒƒ์ด๋‹ค

์ด์ œ ์ด๋ฅผ ์งง๊ฒŒ ํ‘œํ˜„ํ•œ ๊ฒƒ์ด ์•„๋ž˜์™€ ๊ฐ™๋‹ค
\(\sum\limits_{k=1}^n \frac{b-a}{n} f \left(a+ \frac{b-a}{n}k \right)\)

์ง์‚ฌ๊ฐํ˜• ๋„“์ดํ•ฉ์„ ์ ๋ถ„์œผ๋กœ ๋ณ€ํ™˜

\(\lim_{n \to \infty} \sum\limits_{k=1}^n \frac{b-a}{n} f \left(a+ \frac{b-a}{n}k \right)\)
์•ž์„œ ์–˜๊ธฐํ–ˆ๋“ฏ์ด ์ ๋ถ„์€ n๊ฐœ์˜ ์ง์‚ฌ๊ฐํ˜•๋“ค์˜ ๊ฐ ๋„“์ด์˜ ํ•ฉ์˜ ๊ทนํ•œ๊ฐ’์ด๋‹ˆ $\lim_{n\to \infty}$ ํ‘œ์‹œ๋ฅผ ํ•˜์ž
ํ•˜๊ฒŒ ๋˜๋ฉด ์•„๋ž˜ ๊ทธ๋ฆผ์˜ ๊ณผ์ •์„ ๊ฑฐ์นœ๋‹ค

Desktop View

์šฐ๋ฆฌ๊ฐ€ ๊ทธ๋ฆผ์„ ๋ณด๊ณ  ์•Œ ์ˆ˜ ์žˆ๋Š” ๊ฒƒ์€ n์ด ๋ฌดํ•œ์œผ๋กœ ๊ฐ€๊นŒ์›Œ ๊ฐˆ ์ˆ˜๋ก
y=f(x)์œ„์˜ ์‚์ ธ๋‚˜์˜จ ๋ฉด์ ๋“ค์ด ์ค„์–ด๋“ค๋ฉด์„œ ๊ฒฐ๊ตญ ์—†์–ด์ง€๋Š” ๊ฒƒ์ด๋‹ค
๊ทธ๋ง์€ ์ฆ‰์Šจ ๊ฐ๊ฐ์˜ ๋ณด๋ผ์ƒ‰ ์ง์‚ฌ๊ฐํ˜• ๋ฐ‘๋ณ€ ๊ธธ์ด๋„ ๋จธ๋ฆฌ์นด๋ฝ์ฒ˜๋Ÿผ ์–‡์•„์ง€๋ฉฐ ๊ฒฐ๊ตญ 0์ด ๋œ๋‹ค๋Š” ๋ง์ด๋‹ค
(์ฐธ ๋ณด๋ผ์ƒ‰ ์ง์‚ฌ๊ฐํ˜• ๊ฐ ๋ฐ‘๋ณ€์˜ ๊ธธ์ด๋Š” ์ „๋ถ€ n๋“ฑ๋ถ„ ํ•œ ๊ฒƒ์ด๋ผ ๋˜‘๊ฐ™๋‹ค ์•„๊นŒ ์œ„์—์„œ๋„ ๋งํ–ˆ์ง€๋งŒ)

์—ฌ๊ธฐ์„œ ์‹์„ ์ข€ ๋” ๊ฐ„๋‹จํ•˜๊ฒŒ ๋‚˜ํƒ€๋‚ด๊ธฐ ์œ„ํ•ด ์•„๋ž˜ 3๊ฐ€์ง€๋“ค์„ ๊ฐ„์†Œํ™” ์‹œํ‚ค๊ฒ ๋‹ค
๐Ÿ˜— \(\lim \sum_{n \to \infty}\) = \(\int\)

๐Ÿ˜€ \(a+ \frac{b-a}{n}k\) \(\color{red}{\Rightarrow}\) \(x\)

๐Ÿซก \(\frac{b-a}{n}\) \(\color{red}{\Rightarrow}\) \(dx\)

์—ฌ๊ธฐ์„œ k๊ฐ€ 1~n๊นŒ์ง€๋‹ˆ ๋Œ€์ž…ํ•˜๋ฉด ๋ฒ”์œ„๋Š” a~b๋กœ ๋‚˜์˜ค๊ฒ ๋„ค

\(\therefore \int^b_a f(x)dx\)์œผ๋กœ ๋ถ€ํ˜ธ๊ฐ€ ์žˆ๋Š” ์ง์‚ฌ๊ฐํ˜• ๋„“์ด ํ•ฉ์˜ ๊ทนํ•œ๊ฐ’์ด ๋œ๋‹ค
์ฐธ ์œ„์˜ ๊ทธ๋ฆผ์—์„œ๋Š” ๋ฉด์ ์ด ์–‘(+)์˜ ๋ถ€ํ˜ธ์ธ ๋„“์ด์˜ ํ•ฉ์˜ ๊ทนํ•œ๊ฐ’์ธ๋ฐ
์‚ฌ์‹ค ์–‘(+) ์Œ(-) ๋‘˜๋‹ค ์ƒ๊ด€์—†๋‹ค ย ย  ์•„๋ž˜ ๊ทธ๋ฆผ์„ ํ™•์ธํ•˜์ž

Desktop View


3. ๋ฌดํ•œ๊ธ‰์ˆ˜์™€ ์ •์ ๋ถ„์˜ ๊ด€๊ณ„

์—ฌ๊ธฐ์„œ๋Š” ํ‰ํ–‰์ด๋™๊ณผ ์ถ•์†Œ ๋ฐ ํ™•๋Œ€๊ฐ€ ์ค‘์š”ํ•˜๋‹ค
์ด ๊ธ€์—์„œ๋Š” 4๊ฐ€์ง€ ๋„“์ด ๋ฒ”์œ„๊ฐ€ ๋‹ค ๊ฐ™์€ ์‹์„ ๋ณผ ๊ฒƒ์ด๊ณ ,
์šฐ์„  ์•„๋ž˜์˜ ๋ฌดํ•œ๊ธ‰์ˆ˜ ์‹์„ ๊ธฐ์ค€์œผ๋กœ ์‚ผ๊ฒ ๋‹ค
---------------๊ธฐ์ค€์‹-----------------
\(\lim_{n \to \infty}\sum\limits_{k=1}^n f \left( a+ \frac{p}{n}k \right) \frac{p}{n}\)
-----------------------------------------

case 1) ๊ธฐ์ค€์‹๊ณผ ๋„“์ด๊ฐ€ ๊ฐ™์€ ๊ฒฝ์šฐ1(๊ทธ๋ƒฅ ํ‰๋ฒ”ํ•œ ์ •์ ๋ถ„)

์ด ์‹์—์„œ ์ •์ ๋ถ„์‹์œผ๋กœ ๋ฐ”๊ฟ”์ฃผ๊ธฐ ์œ„ํ•ด x์™€ dx๋ฅผ ์•„๋ž˜์™€ ๊ฐ™์ด ์„ค์ •ํ•˜์ž
\(\left( a+\frac{p}{n}k \color{red}{=} x \right)\) ย ย ย  \(\frac{p}{n} \color{red}{\Rightarrow} dx\)

์—ฌ๊ธฐ์„œ k์— 1์ด๋ž‘ n์„ ๋„ฃ์œผ๋ฉด $\infty$๋กœ ๊ฐˆ ๋•Œ,
๊ฐ๊ฐ a, a+p๋กœ ์ˆ˜๋ ดํ•˜๋Š”๋ฐ ์ด๊ฒŒ ์ •์ ๋ถ„์˜ ๋ฒ”์œ„๋‹ค
\(\color{red}{์ค€์‹}\) = \(\int^{a+p}_a f(x)dx\)

case 2) case1๊ณผ ๋„“์ด ํ•ฉ์ด ๊ฐ™์€ ๊ฒฝ์šฐ2(case1์—์„œ ํ‰ํ–‰์ด๋™ํ•จ)

\(\left( \frac{p}{n}k \color{red}{=} x \right)\) ย ย ย  \(\frac{p}{n} \color{red}{\Rightarrow} dx\)

๋งˆ์ฐฌ๊ฐ€์ง€๋กœ k์— 1, n ๊ฐ๊ฐ ๋Œ€์ž…ํ•˜๋ฉด \(\infty\)๋กœ ๊ฐˆ ๋•Œ,
๊ฐ๊ฐ 0, p๋กœ ์ˆ˜๋ ดํ•œ๋‹ค
(๋งจ ์œ„์˜ ๊ธฐ์กด์‹์— ๋Œ€์ž…ํ•˜๋Š” ๊ฒƒ์œผ๋กœ ํ—ท๊ฐˆ๋ฆฌ๋ฉด ์•ˆ๋œ๋‹ค! ์—ฌ๊ธฐ์„œ ํ•ด์•ผํ•œ๋‹ค)

์ž ๊ทธ๋Ÿผ a๊ฐ€ ๋‚จ์•˜๋„ค?? = \(x \to x-a\) ย  (x๋ฐฉํ–ฅ์œผ๋กœ -a๋งŒํผ ํ‰ํ–‰์ด๋™ํ–ˆ๋‹ค)
\(\color{red}{์ค€์‹}\) = \(\int^{p}_0 f(a+x)dx\)

๐Ÿ–๏ธcase1๊ณผ case2 ๋น„๊ต

Desktop View

ํ‰ํ–‰ ์ด๋™ํ•œ ์ฐจ์ด๊ฐ€ ์ž˜ ๋ณด์ด๋„คใ…Ž ๊ทธ๋ž˜์„œ ๋ฉด์ ์€ ๋‘˜๋‹ค ๊ฐ™๋‹ค

โ˜…์ถ”๊ฐ€

case1์—์„œ x์ถ•์œผ๋กœ a๋งŒํผ ์ด๋™ํ•œ ๊ฒƒ์€ \(x \to x + a\) ํ•œ ๊ฒƒ์œผ๋กœ,
\(\color{red}{์ค€์‹}\) = \(\int^{2a+p}_{2a} f(x-a)dx\)

case1์—์„œ x์ถ•์œผ๋กœb๋งŒํผ ์ด๋™ํ•œ ๊ฒƒ์€ \(x \to x + b\) ํ•œ ๊ฒƒ์œผ๋กœ,
\(\color{red}{์ค€์‹}\) = \(\int^{a+p+b}_{a+b} f(x-b)dx\)

\(\color{red}{\therefore}\) case1, 2 ๊ทธ๋ฆฌ๊ณ  ์ด 2๊ฐœ ์ถ”๊ฐ€ ์˜ˆ์‹œ ํ•ฉํ•ด์„œ 4๊ฐœ๋Š” ์ „๋ถ€ ๋ฉด์ ์ด ๊ฐ™๋‹ค

case 3) \(\left( \frac{k}{n} \color{red}{=} x \right)\)๋กœ ์„ค์ •ํ•  ๋•Œ (๊ธฐ์ค€์‹ ํ† ๋Œ€๋กœ)

\(\left( \frac{k}{n} \color{red}{=} x \right)\) ย ย ย  \(\frac{1}{n} \color{red}{\Rightarrow} dx\)
\(\color{red}{์ค€์‹}\) = \(p\int^{1}_{0} f(a+px)dx\)

case 4) \(\left( \frac{k}{2n} \color{red}{=} x \right)\)๋กœ ์„ค์ •ํ•  ๋•Œ

๋งจ ์œ„์˜ ๋ฌดํ•œ๊ธ‰์ˆ˜ ๊ธฐ์ค€์‹์€ ์•„๋ž˜์™€ ๊ฐ™๋‹ค
\(\lim_{n \to \infty}\sum\limits_{k=1}^n f \left( a+ \frac{p}{n}k \right) \frac{p}{n}\)

\(\left( \frac{k}{2n} \color{red}{=} x \right)\) ย ย ย  \(\frac{1}{2n} \color{red}{\Rightarrow} dx\)
์ด๋ ‡๊ฒŒ x์™€ dx๋ฅผ ์„ค์ •ํ•˜๊ณ  ์‹ถ๋‹ค๋ฉด ๋ฌดํ•œ๊ธ‰์ˆ˜ ๊ธฐ์ค€์‹์„ ์•„๋ž˜์™€ ๊ฐ™์ด ๋ณผ ์ˆ˜ ์žˆ์ง€ ์•Š์„๊นŒ
\(\lim_{n \to \infty}\sum\limits_{k=1}^n f \left( a+ 2p\frac{k}{2n} \right) \frac{1}{2n}2p\)

\(\color{red}{์ค€์‹}\) = \(2p\int^{\frac{1}{2}}_{0} f(a+2px)dx\)


๐Ÿ–๏ธcase3๊ณผ case4 ๋น„๊ต

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์œ„ 2๊ฐœ๋Š” ์—ญ์‹œ ๊ฐ™์€ ๊ฒฐ๊ณผ ๊ฐ’์„ ๋„์ถœํ•˜๋Š”๋ฐ ์ฐจ์ด์ ์ด ์žˆ๋‹ค๋ฉด, ย  p์™€ 2p ์ฐจ์ด์ธ๋ฐ ์ถ•์†Œ ๊ฐœ๋…์œผ๋กœ ๋ณด๋ฉด ๋˜๊ฒ ๋‹ค
์ฆ‰ case4)์˜ ํ•จ์ˆ˜๊ฐ€ case 3)์˜ ํ•จ์ˆ˜์—์„œ \(\frac{1}{2}\) ์ถ•์†Œํ•ด์„œ ๊ตฌ๊ฐ„๋„ \(\int^{\frac{1}{2}}_{0}\) ์ด๋ ‡๊ฒŒ ์ ˆ๋ฐ˜์œผ๋กœ ๊ฐ์†Œํ•œ ๊ฒƒ์ด๋‹ค
๊ทธ๋ž˜์„œ case4)์˜ ํ•จ์ˆ˜์—์„œ p์— 2๋ฐฐ๋ฅผ ํ•ด์ค€ ๊ฒƒ์ด๋‹ค
์ถ•์†Œ๋ฅผ ์ž˜ ๋ชจ๋ฅด๊ฒ ์œผ๋ฉด ์‰ฝ๊ฒŒ ์ดํ•ดํ•˜๊ธฐ ์œ„ํ•ด ์•„๋ž˜ sinx์™€ sin2x ๊ทธ๋ž˜ํ”„๋ฅผ ๋ณด๋ฉด ๋˜๊ฒ ๋‹ค

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์ฐธ๊ณ 

  1. [ํ์Šคํ„ฐ๋””] ย ย ย  ์ •์ ๋ถ„๊ณผ ๋ฌดํ•œ๊ธ‰์ˆ˜
  2. [์ˆ˜์•…์ค‘๋…] ย ย ย  ๋ถ€์ •์ ๋ถ„
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