- Let y=y(x) satisfy y(1)=1 and x3+2x2y−y3=2. Find y″.
- Find \displaystyle\left.\frac{d}{dx}2^{\tan x}\right|_{x=\pi/4}.
- Find \displaystyle\lim_{x\to\infty}\left(3x^2+5\right)^{\frac1{\ln x+1}}.
- Given f\left(x\right)=\cos^2x, find f^{\left(2k\right)}\left(0\right).
- Find the arc length of the curve y=\ln\left(1-x^2\right) from x=0 to x=1/4.
- Let g\left(x\right) be the inverse function of f\left(x\right)=x^2e^x for x\geq0. Find \displaystyle\int_0^eg\left(x\right)dx.
- 當 x=0 時有 u=0;
- 當 x=e 時有 u=1;
- 求導有 dx=f'\left(u\right)=\left(2ue^u+u^2e^u\right)du
- Find the tangent plane to the surface \tan\left(xy\right)=\sin\left(yz\right) at the point x=1/4, y=\pi and z=1/2.
- Given f\left(x,y\right)=x^3+2xy+y^3, find the unit vector \vec{u} such that the directional derivative of f\left(x,y\right) at the point \left(1,2\right) and in the direction \vec{u} attains its maximum.
- Evaluate \displaystyle\int_{x=0}^{x=8}\int_{y=x^{1/3}}^{y=2}\sin\left(y^4\right)dydx.
- Solve the differential equation x^3y'=\sec y, y\left(1\right)=\pi/6.
訣竅
運用隱函數微分求解。解法
運用隱函數微分求導可得3x^2+4xy+2x^2y'-3y^2y'=0
取 x=1 並使用 y\left(1\right)=1 有3+4+2y'\left(1\right)-3y'\left(1\right)=0
可解得 y'\left(1\right)=7。進一步地再用一次隱函數微分可得6x+4y+8xy'+2x^2y''-6yy'^2-3y^2y''=0
取 x=1 並使用 y\left(1\right)=1 與 y'\left(1\right)=7 可得6+4+56+2y''\left(1\right)-294-3y''\left(1\right)=0
因此所求為 y''\left(1\right)=-228。訣竅
運用連鎖律進行微分即可。解法
使用連鎖律求導可得\displaystyle\frac{d}{dx}2^{\tan x}=2^{\tan x}\ln 2\cdot\frac{d}{dx}\tan x=2^{\tan x}\ln 2\sec^2x
在 x=\pi/4 取值可得所求為 \displaystyle\left.\frac{d}{dx}2^{\tan x}\right|_{x=\pi/4}=4\ln2。訣竅
運用換底公式與羅必達法則求解。解法
運用換底公式與羅必達法則可計算如下\displaystyle\lim_{x\to\infty}\left(3x^2+5\right)^{\frac1{\ln x+1}}=\lim_{x\to\infty}\exp\left(\frac{\ln\left(3x^2+5\right)}{\ln x+1}\right)=\exp\left(\lim_{x\to\infty}\frac{\ln\left(3x^2+5\right)}{\ln x+1}\right)=\exp\left(\lim_{x\to\infty}\frac{6x/\left(3x^2+5\right)}{1/x}\right)=e^2
訣竅
運用倍角公式與求導之間產生的規律求解。解法
使用二倍角公式可知\displaystyle f\left(x\right)=\cos^2x=\frac{1+\cos2x}2
那麼容易看出當 k\in\mathbb{N} 時有f^{\left(2k\right)}\left(x\right)=\left(-1\right)^k2^{2k-1}\cos2x
故所求為f^{\left(2k\right)}\left(0\right)=\begin{cases}1,&k=0,\\\left(-1\right)^k2^{2k-1},&k\in\mathbb{N}.\end{cases}
訣竅
運用曲線弧長公式求解即可。解法
使用曲線弧長公式如下\displaystyle\begin{aligned}s&=\int_0^{1/4}\sqrt{1+\left(\frac{d}{dx}\ln\left(1-x^2\right)\right)^2}dx=\int_0^{1/4}\sqrt{1+\left(\frac{-2x}{1-x^2}\right)^2}dx=\int_0^{1/4}\frac{1+x^2}{1-x^2}dx\\&=\int_0^{1/4}\left(-1+\frac1{1-x}+\frac1{1+x}\right)dx=\left.\left(-x+\ln\frac{1+x}{1-x}\right)\right|_0^{1/4}=-\frac14+\ln\frac53\end{aligned}
訣竅
利用反函數的定義考慮變數變換。解法
令 x=f\left(u\right),那麼\displaystyle\begin{aligned}\int_0^eg\left(x\right)dx&=\int_0^1u\cdot\left(2ue^u+u^2e^u\right)du=\int_0^1\left(2u^2+u^3\right)de^u=\left(2u^2+u^3\right)e^u\Big|_0^1-\int_0^1\left(4u+3u^2\right)e^udu\\&=3e-\int_0^1\left(4u+3u^2\right)de^u=3e-\left(4u+3u^2\right)e^u\Big|_0^1+\int_0^1\left(4+6u\right)e^udu\\&=-4e+2\int_0^1\left(2+3u\right)de^u=-4e+2\left(2+3u\right)e^u\Big|_0^1-2\int_0^13e^udu\\&=6e-4-6e^u\Big|_0^1=2\end{aligned}
【另解】 讀者可能也可以注意到這樣的計算:
\displaystyle\int_0^eg\left(x\right)dx=\int_0^1\left(2u^2+u^3\right)e^udu=2\int_0^1u^2e^udu+\int_0^1u^3de^u=e-\int_0^1u^2e^udu
同樣使用分部積分法可得如上類似的過程。訣竅
運用梯度求出曲面的法向量,隨後使用點法式寫出切平面方程式。解法
設 F\left(x,y,z\right)=\tan\left(xy\right)-\sin\left(yz\right),如此計算其梯度有\nabla F\left(x,y,z\right)=\left(y\sec^2\left(xy\right),x\sec^2\left(xy\right)-z\cos\left(yz\right),-y\cos\left(yz\right)\right)
因此曲面 F\left(x,y,z\right)=0 在 \left(1/4,\pi,1/2\right) 處的法向量為 \nabla F\left(1/4,\pi,1/2\right)=\left(2\pi,1/2,0\right)。故由點法式可知切平面方程式為\displaystyle2\pi\left(x-\frac14\right)+\frac12\left(y-\pi\right)+0\left(z-\frac12\right)=0
或寫為 4\pi x+y=2\pi。訣竅
因為函數沿著其梯度的方向可使方向導數達到最大值,故求其梯度即可。解法
直接計算梯度有\nabla f\left(x,y\right)=\left(3x^2+2y,2x+3y^2\right)
而在 \left(1,2\right) 處的梯度為 \nabla f\left(1,2\right)=\left(7,14\right)\parallel\left(1,2\right),故取 \displaystyle\vec{u}=\frac{\left(1,2\right)}{\sqrt5}。訣竅
交換積分次序求解即可。解法
原積分範圍 \left\{\begin{aligned}&0\leq x\leq8\\&x^{1/3}\leq y\leq2\end{aligned}\right. 可改寫為 \left\{\begin{aligned}&0\leq x\leq y^3\\&0\leq y\leq2\end{aligned}\right.,如此所求的重積分可改寫並計算如下\displaystyle\int_{x=0}^{x=8}\int_{y=x^{1/3}}^{y=2}\sin\left(y^4\right)dydx=\int_0^2\int_0^{y^3}\sin\left(y^4\right)dxdy=\int_0^2y^3\sin\left(y^4\right)dy=\left.-\frac{\cos\left(y^4\right)}4\right|_0^2=\frac{1-\cos16}4
訣竅
運用分離變量法求解即可。解法
運用分離變量法改寫如下\displaystyle\cos ydy=\frac{dx}{x^3}
兩邊在 \left[1,x\right] 上同取積分可得\displaystyle\sin\left(y\left(x\right)\right)-\sin\left(y\left(1\right)\right)=-\frac1{2x^2}+\frac12=\frac{x^2-1}{2x^2}
因此所求為\displaystyle y\left(x\right)=\sin^{-1}\left(\frac{2x^2-1}{2x^2}\right)
- Evaluate \displaystyle\int_{y=0}^{y=2/3}\int_{x=y}^{x=2-2y}\left(x+2y\right)e^{y-x}dxdy by making the change of variable u=x+2y and v=y-x.
- Use Lagrange multipliers to find the minimal distance from the origin to the surface z^2=xy+1.
- 若 z=0,則第四式寫為 xy=-1,或 y=-x^{-1},從而第一式與第二式分別為 2x-\lambda x^{-1}=0 與 -2x^{-1}+\lambda x=0,皆乘以 x 則有 2x^2=\lambda、\lambda x^2=2,故有 2x^4=2,至此解得 x=\pm1,從而 y=\mp1。
- 若 \lambda=1,則第一式與第二式寫為 2x+y=0 與 2y+x=0,故得 \left(x,y\right)=\left(0,0\right),那麼第四式則給出 z=\pm1。
訣竅
按照題意使用變數變換,其中應留意積分範圍的改寫與 Jacobian 行列式的計算;亦可直接計算重積分。解法一
按照題意中的變數變換,容易解得 \displaystyle x=\frac{u-2v}3、\displaystyle y=\frac{u+v}3,如此積分範圍 \left\{\begin{aligned}&y\leq x\leq2-2y\\&0\leq y\leq2/3\end{aligned}\right. 可以表達為\displaystyle\frac{u+v}3\leq\frac{u-2v}3\leq\frac{6-2u-2v}3,\qquad0\leq\frac{u+v}3\leq\frac23
即 \left\{\begin{aligned}&0\leq u\leq2\\&-u\leq v\leq0\end{aligned}\right.。再者其 Jacobian 行列式可計算如下\displaystyle\frac{\partial\left(x,y\right)}{\partial\left(u,v\right)}=\left|\begin{matrix}\displaystyle\frac{\partial x}{\partial u}&\displaystyle\frac{\partial x}{\partial v}\\\displaystyle\frac{\partial y}{\partial u}&\displaystyle\frac{\partial y}{\partial v}\end{matrix}\right|=\left|\begin{matrix}\displaystyle\frac13&\displaystyle-\frac23\\\displaystyle\frac13&\displaystyle\frac13\end{matrix}\right|=\frac13
據此所求的重積分可改寫並計算如下\displaystyle\begin{aligned}\int_{y=0}^{y=2/3}\int_{x=y}^{x=2-2y}\left(x+2y\right)e^{y-x}dxdy&=\int_0^2\int_{-u}^0ue^v\cdot\frac13dvdu\\&=\frac13\int_0^2u\left(1-e^{-u}\right)du=\left.\frac13\left(ue^{-u}+e^{-u}+\frac{u^2}2\right)\right|_0^2=\frac{3e^{-2}+1}3\end{aligned}
解法二【本題不可採用此法】
直接計算重積分有\displaystyle\begin{aligned}\int_{y=0}^{y=2/3}\int_{x=y}^{x=2-2y}\left(x+2y\right)e^{y-x}dxdy&=-\int_0^{2/3}e^y\int_y^{2-2y}\left(x+2y\right)de^{-x}dy\\&=-\int_0^{2/3}\left[\left(x+2y\right)e^{y-x}\Big|_{x=y}^{x=2-2y}-e^y\int_y^{2-2y}e^{-x}dx\right]dy\\&=-\int_0^{2/3}\left(3e^{3y-2}-3y-1\right)dy=\\&=\left.\left(-e^{3y-2}+\frac{3y^2}2+y\right)\right|_0^{2/3}\\&=\frac13+e^{-2}=\frac{3e^{-2}+1}3\end{aligned}
訣竅
按題意使用拉格朗日乘子法求條件極值;亦可使用初等不等式求極值。解法一
考慮與原點的距離平方函數 f\left(x,y,z\right)=x^2+y^2+z^2,那麼設定拉格朗日乘子函數如下F\left(x,y,z,\lambda\right)=x^2+y^2+z^2+\lambda\left(xy-z^2+1\right)
據此解下列的聯立方程組\left\{\begin{aligned}&F_x\left(x,y,z,\lambda\right)=2x+\lambda y=0\\&F_y\left(x,y,z,\lambda\right)=2y+\lambda x=0\\&F_z\left(x,y,z,\lambda\right)=2z-2\lambda z=0\\&F_{\lambda}\left(x,y,z,\lambda\right)=xy-z^2+1=0\end{aligned}\right.
由第三式可知 z=0 或 \lambda=1。\left(1,-1,0\right),\quad\left(-1,1,0\right),\quad\left(0,0,1\right),\quad\left(0,0,-1\right)
故直接驗算最近的距離為 1,座標為 \left(0,0,\pm1\right)。解法二【本題不可採用此法】
由於 \left(x+y\right)^2\geq0,故 \displaystyle xy\geq-\frac{x^2+y^2}2,如此曲面 z^2=xy+1 上的點與原點的距離有如下的估算\displaystyle d=\sqrt{x^2+y^2+z^2}=\sqrt{x^2+xy+y^2+1}\geq\sqrt{\frac{x^2+y^2}2+1}=1
其中等號成立條件為 \left(x,y\right)=\left(0,0\right),此時 z=\pm1,故最近點座標為 \left(0,0,\pm1\right)。
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