Post

derivative

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>
0. ์•Œ์•„์•ผ ํ•  ๊ฒƒ

1. ์‚ผ๊ฐํ•จ์ˆ˜์˜ ๋„ํ•จ์ˆ˜ 
 1-1 (1)์˜ ์ฆ๋ช…
 1-2 (2)์˜ ์ฆ๋ช…
 1-3 (3)์˜ ์ฆ๋ช…
 1-4 (4)์˜ ์ฆ๋ช…
 1-5 (5)์˜ ์ฆ๋ช…
 1-6 (6)์˜ ์ฆ๋ช…
 1-7 ์œ„์— ๋ฏธ๋ถ„ํ•œ ๊ฒฐ๊ณผ๋“ค์„ ์ ๋ถ„ํ•˜๋ฉด?
 
2. ์ง€์ˆ˜๋กœ๊ทธํ•จ์ˆ˜์˜ ๋„ํ•จ์ˆ˜
 2-1. (2) ์ฆ๋ช…
 2-2. (1) ์ฆ๋ช…
 2-3. (4) ์ฆ๋ช…
 2-4. (3) ์ฆ๋ช…

3. ์†๋„์™€ ๊ฐ€์†๋„ 
 
4. ๋ฏธ๋ถ„ ๊ฐ„๋‹จํ•œ ์˜ˆ์‹œ

5. ์ ๋ถ„ ๊ฐ„๋‹จํ•œ ์˜ˆ์‹œ

0. ์•Œ์•„์•ผ ํ•  ๊ฒƒ

โ€ป๋กœ๊ทธ์˜ ์ง€์ˆ˜๋Š” ํ•ญ์ƒ +(์–‘)๋ถ€ํ˜ธ๋‹ค
โ€ป๋ชซ์˜ ๋ฏธ๋ถ„ (red)(๋งค์šฐ ์ค‘์š”ํ•˜๋‹ค)
\(\left( \frac{f(x)}{g(x)} \right)' = \frac{f'(x) \cdot g(x) - f(x) \cdot g'(x)}{ \left\{ g(x) \right\}^2 }\)

ex) \(\left( \frac{1}{x} \right)' = \frac{0 \cdot x -1 \cdot 1}{x^2} = \frac{-1}{x^2}\)

1. ์‚ผ๊ฐํ•จ์ˆ˜์˜ ๋„ํ•จ์ˆ˜

(1) \((sinx)'= cosx\)
(2) \((cosx)' = -sinx\)
(3) \((tanx)' = sec^2 x\)
(4) \((cotx)'=-csc^2 x\)
(5) \((secx)' = secx \cdot tanx\)
(6) \((cscx)' = -cscx \cdot cotx\)
์ด๊ฑฐ ์‰ฝ๊ฒŒ ์™ธ์šฐ๋Š” ๋ฒ• \(\color{red}{\Rightarrow}\) c๋กœ ์‹œ์ž‘ํ•˜๋Š”๋ฐ์„œ ๋ฏธ๋ถ„ํ•˜๋Š” ๊ฒƒ์€ -๋ถ€ํ˜ธ๊ฐ€ ๋ถ™๋„ค

1-1. (1)์˜ ์ฆ๋ช…

\(f(x)=sinx\)

\((sinx)' = f'(x)=\lim_{h\to0} \frac{f(x+h)-f(x)}{h} = \lim_{h\to0} \frac{sin(x+h)-sin(x)}{h}\)
์—ฌ๊ธฐ์„œ x+h \(\color{red}{\Rightarrow}\) A, ย ย ย  x \(\color{blue}{\Rightarrow}\) B๋ผ ์„ค์ •ํ•˜๊ฒ ๋‹ค
์‚ผ๊ฐํ•จ์ˆ˜ ๋ง์…ˆ๊ณต์‹์„ ์‚ฌ์šฉํ•˜์ž. ์•„๋ž˜๋Š” $sin(A-B)$์—์„œ ์œ„์น˜๋งŒ ์กฐ๊ธˆ ๋ฐ”๊พผ ๊ฒƒ์ด๋‹ค ๊ฒฐ๊ณผ๋Š” ๊ฐ™๋‹ค

๐Ÿผ์ฐธ๊ณ  \(sin(A)-sin(B)=2cos \left( \frac{A+B}{2} \right) sin \left( \frac{A-B}{2} \right)\)
๊ทธ๋Ÿผ ์œ„์—์„œ ์ •ํ•œA, B๋ฅผ ์—ฌ๊ธฐ์— ๋Œ€์ž…ํ•ด๋ณด๋ฉด ์–ด๋–ค ์‹์ด ๋˜ ์‚ฐ์ถœ๋˜์ง€?

์œ„์˜ ์‚ผ๊ฐํ•จ์ˆ˜ sin๊ณต์‹์„ ์ด์šฉํ•˜์—ฌ \(\lim_{h\to0} \frac{sin(x+h)-sin(x)}{h}\)์„ ๋‹ค์‹œ ์ „๊ฐœํ•˜๋ฉด ์•„๋ž˜์™€ ๊ฐ™๋‹ค
\(\lim_{h\to0} \frac{2cos(x+\frac{h}{2}) \cdot sin(\frac{h}{2})}{h} = \frac{cos(x+\frac{h}{2}) \cdot sin(\frac{h}{2})}{\frac{h}{2}}\)

์ด๋ ‡๊ฒŒ ๋ณ€ํ˜•์‹œํ‚ค๋ฉด ์šฐ์ธก ๊ทนํ•œ๊ฐ’์ธ sin์ชฝ์€ 1๋กœ ์ˆ˜๋ ดํ•˜๊ณ ,
์ขŒ์ธก ๊ทนํ•œ๊ฐ’์ธ cos์ชฝ์—” \(cosx\) ๋กœ ์ˆ˜๋ ดํ•œ๋‹ค

1-2. (2)์˜ ์ฆ๋ช…

\(f(x) = cosx\)

\((cosx)'=f'(x)=\lim_{h\to0} \frac{cos(x+h) - cos(x)}{h}\)
์—ฌ๊ธฐ์„œ 2x+h๋ฅผ A+B, ย ย  h๋ฅผ A-B๋กœ ๋ณด์ž
๊ทธ๋ฆฌ๊ณ  ์•„๋ž˜์‹ ์ฐธ๊ณ 
\(cos(A)-cos(B) \\ =-2sin \left( \frac{A+B}{2} \right) sin \left( \frac{A-B}{2} \right)\)

์ด์–ด์„œ ์œ„์˜ ์‹์— ๋Œ€์ž…ํ•˜๋ฉด \(\lim_{h\to0} \frac{-2sin(x+\frac{h}{2}) \cdot sin(\frac{h}{2})}{h} = \lim_{h\to0} \frac{-sin(x+\frac{h}{2}) \cdot sin(\frac{h}{2})}{\frac{h}{2}}\)
์ด๋ ‡๊ฒŒ ๋˜๋ฉด ์ „๋ถ€ ์ˆ˜๋ ดํ•˜๊ณ  ๋‚จ๋Š” ๊ฒƒ์€ \(-(sinx \cdot 1) \cdot 1 \Rightarrow -sinx\)

1-3. (3)์˜ ์ฆ๋ช…

\(f(x) = tan(x)\)

\(=\left( \frac{sin(x)}{cos(x)} \right)' = \frac{(sinx)\ \cdot cosx - sinx \cdot (cosx)'}{cos^2 x} = \frac{cos^2 x + sin^2 x}{cos^2 x} = \frac{1}{cos^2 x} \\ \therefore sec^2 x\)

1-4. (4)์˜ ์ฆ๋ช…

\(f(x) = (cotx)'\)

์ž cot์ด ๋ญ๋ƒ? ย  ๋ฐ”๋กœ \(\frac{1}{tanx}\)์ง€ ์•Š๋А๋ƒ
\(\color{pink}{\Rightarrow}\) \(\frac{(-sinx) \cdot sinx - cosx \cdot cosx}{sin^2 x} = \frac{-1}{sin^2 x} = -csc^2 x\)

1-5. (5)์˜ ์ฆ๋ช…

\(f(x)= (secx)'\)

\(=\left( \frac{1}{cos(x)} \right)' = \frac{0 \cdot cosx -1 \cdot(-sinx)}{cos^2 x} = \frac{1}{cosx} \cdot \frac{sinx}{cosx} = secx \cdot tanx\)
โ€ป์‚ผ๊ฐํ•จ์ˆ˜๋ฅผ ๋ฏธ๋ถ„ํ–ˆ์„ ๋•Œ ๊ฐ์€ ๊ทธ๋Œ€๋กœ ๋‚˜์˜จ๋‹ค! ย ย  ex) \(3x\)
ex-1) \((sec3x)' = 3 \cdot sec3x \cdot tan3 x\)
ex-2) \((tan6x)' =6 \cdot sec^2 6x\)

1-6. (6)์˜ ์ฆ๋ช…

\(f(x)= (csc(x))'\)

๋ฐ”๊ฟ”์“ฐ๋ฉด \(\left( \frac{1}{sin(x)} \right)'\) ์ด๋ ‡๊ฒŒ ๋˜๋Š”๋ฐ ์—ฌ๊ธฐ์„œ ํ•ฉ์„ฑํ•จ์ˆ˜ ๋ฏธ๋ถ„๊ณต์‹ ์“ฐ์ž \(\left( \frac{f(x)}{g(x)} \right)'\)
\(\Rightarrow\) \(\frac{0x \cdot sinx - 1 \cdot cosx}{sin^2x} = \frac{-1}{sinx} \cdot \frac{cosx}{sinx} = -cscx \cdot cotx\)

1-7. ์œ„์— ๋ฏธ๋ถ„ํ•œ ๊ฒฐ๊ณผ๋“ค์„ ์ ๋ถ„ํ•˜๋ฉด?

(1) \(\int cosx dx = sinx + C\)
(2) \(\int sinx dx = -cosx + C\)
(3) \(\int sec^2xdx = tanx + C\)
(4) \(\int csc^2xdx = -cotx+C\)
(5) \(\int secx \cdot tanxdc = secx+C\)
(6) \(\int cscx \cdot cotxdx = -csc + C\)



2. ์ง€์ˆ˜ $\cdot$ ๋กœ๊ทธํ•จ์ˆ˜์˜ ๋„ํ•จ์ˆ˜

(1) \(\left( e^x \right)' = e^x\)
(2) \(\left( a^x \right)' = a^x lna\)
(3) \(\left( lnx \right)' = \frac{1}{x}\)
(4) \(\left( log_ax \right)' = \frac{1}{x} \cdot \frac{1}{lna}\)
์•„๋ž˜์— ์ดํ•ดํ•˜๊ธฐ ์‰ฝ๊ฒŒ (2) โ†’ (1) โ†’ (4) โ†’ (3) ์ˆœ์œผ๋กœ ์ฆ๋ช…ํ•˜๊ฒ ๋‹ค

2-1. (2) ์ฆ๋ช…

\(f'(x) = \left( a^x \right)' = \lim_{h\to0} \frac{f(x+h)-f(x)}{h} = lim_{h\to0} \frac{a^{x+h}-a^x}{h}\)

๐ŸŽฒ์ฐธ๊ณ 
\(lim_{x\to0} \frac{a^{px}-1}{qx} = \frac{p}{q} \cdot lna\)
\(lim_{x\to0} \frac{a^{x}-1}{x} = lna\)

์ด๊ฑธ ์ฐธ๊ณ ํ•ด์„œ ์ด์–ด์„œ ์ „๊ฐœํ•˜๋ฉด
\(\lim_{h\to0} \frac{a^h-1}{h} \cdot a^x \Rightarrow a^x \cdot lna\)

\(\color{red}{ex)}\)
\(\left( 3^x \right)' = 3^x \cdot ln3\)
\(\left( 3^{2x} \right)' = \frac{2}{1} \cdot 3^{2x} \cdot ln3\)

2-2. (1) ์ฆ๋ช…

\(\left( e^{x} \right)' = e^x \cdot ln_e e = e^x\)

\(\color{red}{ex)}\)
\(\left( e^{3x} \right)' = 3e^{3x} \cdot ln_e e = 3e^{3x}\)

2-3. (4) ์ฆ๋ช…

\(f(x)=log_ax\) ๋ผ๊ณ  ํ•˜์ž
\(\left( log_ax \right)' = f'(x) = lim_{h\to0} \frac{log_a(x+h) - log_a x}{h}\) ์—ฌ๊ธฐ์„œ ๋ถ„๋ชจ๋ฅผ ์—†์• ๊ณ  ๋ถ„์ž๋งŒ ๋ณด์ž

๐Ÿ–๏ธ์ž ๊น ๋ฐ‘์ด ๊ฐ™์€ ๋กœ๊ทธ๋ฅผ ๋นผ๋ฉด? \(\color{red}{\Rightarrow}\) \(log_a A- log_b B = log_a \frac{A}{B}\)
์ž ์ด๋ฅผ ํ™œ์šฉํ•˜๋ฉด \(log_a\left(\frac{x+h}{x} \right)\)๋กœ ๋‚˜ํƒ€๋‚ผ ์ˆ˜ ์žˆ๋„ค? ย  ์ž ๊ทธ๋Ÿฌ๋ฉด ์•„๋ž˜์—์„œ๋Š” x๋ฅผ ๋‚˜๋ˆ„๊ณ  ์ด์–ด์„œ ์ „๊ฐœํ•˜๊ฒ ๋‹ค

\(= \lim_{h\to0} \frac{1}{h} \cdot log_a (1+\frac{h}{x})^{\frac{1}{h}}\) (red) \(=\) \(\lim_{h\to0} log_a (1+\frac{h}{x})^{\frac{x}{h} \cdot \frac{1}{x}}\)

์–ด? ๊ฐ€๋งŒ๋ณด๋‹ˆ \((1+\frac{h}{x})^{\frac{x}{h}} = e\) ๋„ค? \(log_a e^{\frac{1}{x}} = \frac{1}{x}log_a e = \frac{1}{x}\cdot\frac{1}{log_e a}\)
\(\color{purple}{\Rightarrow}\) ย  \(\frac{1}{x} \cdot \frac{1}{lna}\)

โ˜†์ด๊ฒƒ๋„ ์ฐธ๊ณ ํ•˜์ž
ex-1) \(log_a s = \frac{1}{log_s a}\)
ex-2) \(\left(log_2 x \right)' = \frac{1}{x} \cdot \frac{1}{ln2}\)
ex-3) \(\left( log_2 3x \right)' = 3 \cdot \frac{1}{3x}\cdot \frac{1}{1n2}\)
ex-4) \(\left( log_a f(x) \right)' = \frac{f'(x)}{f(x)} \cdot \frac{1}{lna}\)
ex-5) \(\left( log_2 3^{2x} \right)' = \left( 2x \cdot log_2 3 \right)'\) \(\color{pink}{\Rightarrow}\) \(2 \cdot log_2 3\)

2-4. (3) ์ฆ๋ช…

\(f'\left( x \right) = \left( log_e x \right)' = \frac{1}{x} \cdot \frac{1}{ln_e e} = \frac{1}{x}\)

ex) \(ln(\Delta)' = \frac{\Delta'}{\Delta}\) \(\color{pink}{\Rightarrow}\) \(\{ ln(x^2+x+1) \}' = \frac{\{2x+1\} \cdot ln_e e}{\{x^2+x+1\} \cdot ln_e e} = \frac{2x+1}{x^2+x+1}\)

3. ์†๋„์™€ ๊ฐ€์†๋„

๋ฉ”์ดํ”Œ์Šคํ† ๋ฆฌ์˜ ์™€์ผ๋“œ๋ณด์–ด๊ฐ€ ์›€์ง์ธ๋‹ค๊ณ  ์ƒ๊ฐํ•ด๋ณด์ž
์•„๋ž˜๋Š” Cartesianย coordinate system์—์„œ \((x+k)x^2\) ์„ ๋‚˜ํƒ€๋‚ธ ๊ฒƒ์ด๋‹ค
Desktop View

์•„๋ž˜๋Š” ์™€์ผ๋“œ ๋ณด์–ด๊ฐ€ ๋„๋‹ฌํ–ˆ๋˜ ๊ตฌ๊ฐ„๋“ค์„ ์ž„์˜๋กœ x์ถ•, y์ถ•์„ ์ด์šฉํ•ด ๋‚˜ํƒ€๋‚ธ ๊ฒƒ์ด๋‹ค
Desktop View


์ผ๋ฐ˜์ ์œผ๋กœ ์ขŒํ‘œํ‰๋ฉด ์œ„๋ฅผ ์›€์ง์ด๋Š” ์  P์˜ t(์‹œ๊ฐ„)์—์„œ์˜ ์œ„์น˜(x,y)๋Š”
t๋ฅผ ๋งค๊ฐœ๋ณ€์ˆ˜๋กœ ํ•˜๋Š” ๋‘ ํ•จ์ˆ˜ \(x=f(t)\), ย  \(y=g(t)\)๋กœ ๋‚˜ํƒ€๋‚ผ ์ˆ˜ ์žˆ๋‹ค
์ฐธ๊ณ ๋กœ \(`t(์‹œ๊ฐ„) >0`\) ๋‹น์—ฐํ•œ ๊ฒƒ์ด๋‹ค

๋‹ค์Œ์€ ์™€์ผ๋“œ๋ณด์–ด๊ฐ€ \(p_1(1, 6)\)์—์„œ \(P_?(2, 17)\)๊นŒ์ง€ ์ด๋™ํ–ˆ์„ ๋•Œ
t์ดˆ ๊ฑธ๋ฆฐ ๊ฒƒ์— ๋Œ€ํ•œ ๊ทธ๋ž˜ํ”„๋ฅผ ๋‚˜ํƒ€๋‚ธ ๊ฒƒ์ด๋‹ค.
Desktop View
์ด๋•Œ ์  P์˜ t(์‹œ๊ฐ„)์—์„œ์˜ ์†๋„์™€ ์†๋ ฅ, ๊ฐ€์†๋„์™€ ๊ฐ€์†๋„์˜ ํฌ๊ธฐ๋Š” ๋‹ค์Œ๊ณผ ๊ฐ™๋‹ค
(1) ์  P์—์„œ t(์‹œ๊ฐ„)์—์„œ์˜ ์†๋„์™€ ์†๋ ฅ
(2) ์†๋„: ( \(f'(t), g'(t)\) )
(3) ์†๋ ฅ: \(\sqrt{f'(t)^2 + g'(t)^2}\)
*๊ฑฐ๋ฆฌ: ย  \(t \cdot ์†๋ ฅ\)
์ˆœ๊ฐ„์ ์ธ x์ถ•๊ณผ y์ถ•์˜ ๋ณ€ํ™”์œจ์— ๋Œ€ํ•ด \(\frac{dx}{dt} = f'(t)\)์™€ \(\frac{dy}{dt} = g'(t)\) ๋กœ ํ‘œํ˜„ํ–ˆ๋‹ค.

์ž ๊ทธ๋Ÿฌ๋ฉด \(p_1 \rightarrow p_?\) ๊ตฌ๊ฐ„์„ ์ง€๋‚˜๊ฐˆ ๋•Œ 3์ดˆ ์ผ๋•Œ์˜ ์†๋ ฅ์€ ์–ด๋–จ์ง€ ๊ณ„์‚ฐํ•ด๋ณด์ž
ํ”ผํƒ€๊ณ ๋ผ์Šค ์ •๋ฆฌ๋ฅผ ์—ฐ์ƒํ•˜๋ฉด ์‰ฝ๋‹ค
์šฐ์„  ๋‘์ ์‚ฌ์ด ๊ฑฐ๋ฆฌ ๊ตฌํ•˜๋Š” ๊ณต์‹์ธ ์œ ํด๋ฆฌ๋“œ ๊ฑฐ๋ฆฌ ๊ณต์‹\(\sqrt{(x_2-x_1)^2 + (y_2-y_1)^2}\) ์„ ์ด์šฉํ•ด์„œ
\(\sqrt{122}\)๊ฐ€ ๋‚˜์™”๊ณ  ๊ทธ ๊ตฌ๊ฐ„์—์„œ์˜ 3์ดˆ๊ฐ€ ๊ฒฝ๊ณผํ–ˆ์„ ๋•Œ ์†๋ ฅ์€ \(\frac{\sqrt{122}}{3}\)์ด๋‹ค

๊ทธ๋ ‡๋‹ค๋ฉด ๊ฐ€์†๋ ฅ์€ ์–ด๋–ป๊ฒŒ??
(๋ฏธ๋ถ„ 2๋ฒˆ์ด๋‹ค)

(1) ๊ฐ€์†๋„: \((f''(t), g''(t))\)
(2) ๊ฐ€์†๋„์˜ ํฌ๊ธฐ: \(\sqrt{f''(t)^2 + g''(t)^2}\)

4. ๋ฏธ๋ถ„ ๊ฐ„๋‹จํ•œ ์˜ˆ์‹œ

ํ•จ์„ฑํ•จ์ˆ˜์˜ ๋ฏธ๋ถ„: \((\clubsuit \cdot \Delta)'\) \(\color{pink}{\Rightarrow}\) \(\clubsuit' \cdot \Delta + \clubsuit \cdot \Delta'\)

ex-1) \(f(x) = e^x \cdot sin5x\)
\(f'(x) = e^x sin5x + e^x (5cos5x)\)

ex-2) \(f(x) = e^{-2x}cos3x\)
\(f'(x) = (-2 \cdot e^{-2x})cos3x + e^{-2x}(-3 \cdot sin3x) \\ = -e^{-2x}(2 \cdot cos3x + 3 \cdot sin3x)\)

5. ์ ๋ถ„ ๊ฐ„๋‹จํ•œ ์˜ˆ์‹œ

ex-1) \(\{ ln(x^2+1) \}' = \frac{2x}{x^2+1}\) ์ด๊ฒƒ์€ ๋ฏธ๋ถ„๋œ ๊ฐ’์ด๋‹ค
๊ทธ๋Ÿผ ์ ๋ถ„์€? ย  \(\int \frac{2x}{x^2+1}dx = ln(x^2+1) + C\)

ex-2) \(\int tanx dx\)
\(= \int \frac{sinx}{cosx}dx = -\int \frac{-sinx}{cosx}dx \Rightarrow -ln |cosx| +C\)

ex-3) $\int e^{3x} dx = \frac{1}{3}e^{3x} + C$

ex-4) $\int cos3xdx = \frac{1}{3} sin3x + C$


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