张宇1000题高等数学 第五章 一元函数微分学的应用(一)——几何应用

目录

  • A A A
    • 4.曲线 r = cos ⁡ 2 θ r=\cos2\theta r=cos2θ θ = π 4 \theta=\cfrac{\pi}{4} θ=4π处的切线方程为______。
  • B B B
    • 6.设 f ( x ) = ∣ x ( 3 − x ) ∣ f(x)=|x(3-x)| f(x)=x(3x),则(  )
      ( A ) x = 0 (A)x=0 (A)x=0 f ( x ) f(x) f(x)的极值点,但 ( 0 , 0 ) (0,0) (0,0)不是曲线 y = f ( x ) y=f(x) y=f(x)的拐点;
      ( B ) x = 0 (B)x=0 (B)x=0不是 f ( x ) f(x) f(x)的极值点,但 ( 0 , 0 ) (0,0) (0,0)是曲线 y = f ( x ) y=f(x) y=f(x)的拐点;
      ( C ) x = 0 (C)x=0 (C)x=0 f ( x ) f(x) f(x)的极值点,且 ( 0 , 0 ) (0,0) (0,0)是曲线 y = f ( x ) y=f(x) y=f(x)的拐点;
      ( D ) x = 0 (D)x=0 (D)x=0不是 f ( x ) f(x) f(x)的极值点,且 ( 0 , 0 ) (0,0) (0,0)也不是曲线 y = f ( x ) y=f(x) y=f(x)的拐点。
    • 14.设函数 f ( x ) f(x) f(x)可导,且满足 x f ′ ( x ) = f ′ ( − x ) + 1 , f ( 0 ) = 0 xf'(x)=f'(-x)+1,f(0)=0 xf(x)=f(x)+1,f(0)=0,求:
      • (1) f ′ ( x ) f'(x) f(x)
      • (2)函数 f ( x ) f(x) f(x)的极值。
  • C C C
    • 10.设 f ( x ) = { lim ⁡ n → ∞ 1 n ( 1 + cos ⁡ x n + cos ⁡ 2 x n + ⋯ + cos ⁡ n − 1 n x ) , x > 0 , 1 , x = 0 , f ( − x ) , x < 0. f(x)=\begin{cases}\lim\limits_{n\to\infty}\frac{1}{n}\left(1+\cos\frac{x}{n}+\cos\frac{2x}{n}+\cdots+\cos\frac{n-1}{n}x\right),&x>0,\\1,&x=0,\\f(-x),x<0.\end{cases} f(x)=nlimn1(1+cosnx+cosn2x++cosnn1x),1,f(x),x<0.x>0,x=0,
      • (1)求 f ′ ( 0 ) f'(0) f(0)
      • (2)求 f ( x ) f(x) f(x) [ − π , π ] [-\pi,\pi] [π,π]上的最大值。
  • 写在最后

A A A

4.曲线 r = cos ⁡ 2 θ r=\cos2\theta r=cos2θ θ = π 4 \theta=\cfrac{\pi}{4} θ=4π处的切线方程为______。

  曲线参数方程 { x = cos ⁡ 2 θ cos ⁡ θ , y = cos ⁡ 2 θ sin ⁡ θ , θ = π 4 \begin{cases}x=\cos2\theta\cos\theta,\\y=\cos2\theta\sin\theta,\end{cases}\theta=\cfrac{\pi}{4} { x=cos2θcosθ,y=cos2θsinθ,θ=4π对应 ( x 0 , y 0 ) = ( 0 , 0 ) (x_0,y_0)=(0,0) (x0,y0)=(0,0)
d y d x ∣ θ = π 4 = − 2 sin ⁡ 2 θ sin ⁡ θ + cos ⁡ 2 θ cos ⁡ θ − 2 sin ⁡ 2 θ cos ⁡ θ − cos ⁡ 2 θ sin ⁡ θ ∣ θ = π 4 = 1 , \cfrac{\mathrm{d}y}{\mathrm{d}x}\biggm\vert_{\theta=\frac{\pi}{4}}=\cfrac{-2\sin2\theta\sin\theta+\cos2\theta\cos\theta}{-2\sin2\theta\cos\theta-\cos2\theta\sin\theta}\biggm\vert_{\theta=\frac{\pi}{4}}=1, dxdyθ=4π=2sin2θcosθcos2θsinθ2sin2θsinθ+cos2θcosθθ=4π=1,
  切线方程为 y = x y=x y=x。(这道题主要利用了参数方程求解

B B B

6.设 f ( x ) = ∣ x ( 3 − x ) ∣ f(x)=|x(3-x)| f(x)=x(3x),则(  )
( A ) x = 0 (A)x=0 (A)x=0 f ( x ) f(x) f(x)的极值点,但 ( 0 , 0 ) (0,0) (0,0)不是曲线 y = f ( x ) y=f(x) y=f(x)的拐点;
( B ) x = 0 (B)x=0 (B)x=0不是 f ( x ) f(x) f(x)的极值点,但 ( 0 , 0 ) (0,0) (0,0)是曲线 y = f ( x ) y=f(x) y=f(x)的拐点;
( C ) x = 0 (C)x=0 (C)x=0 f ( x ) f(x) f(x)的极值点,且 ( 0 , 0 ) (0,0) (0,0)是曲线 y = f ( x ) y=f(x) y=f(x)的拐点;
( D ) x = 0 (D)x=0 (D)x=0不是 f ( x ) f(x) f(x)的极值点,且 ( 0 , 0 ) (0,0) (0,0)也不是曲线 y = f ( x ) y=f(x) y=f(x)的拐点。

  由于 f ( x ) = ∣ x ( 3 − x ) ∣ ⩾ 0 , f ( 0 ) = 0 f(x)=|x(3-x)|\geqslant0,f(0)=0 f(x)=x(3x)0,f(0)=0,可知 x = 0 x=0 x=0 f ( x ) f(x) f(x)的极小值点。由 f ( x ) = { 3 x − x 2 , 0 < x < 3 , − 3 x + x 2 , x ⩽ 0 或 x ⩾ 3 , f(x)=\begin{cases}3x-x^2,&0f(x)={ 3xx2,3x+x2,0<x<3,x0x3,可得
f ′ ( x ) = { 3 − 2 x , 0 < x < 3 , − 3 + 2 x , x < 0 或 x > 3. f ′ ′ ( x ) = { − 2 , 0 < x < 3 , 2 , x < 0 或 x > 3. f'(x)=\begin{cases}3-2x,&03.\end{cases}\\ f''(x)=\begin{cases}-2,&03.\end{cases}\\ f(x)={ 32x,3+2x,0<x<3,x<0x>3.f(x)={ 2,2,0<x<3,x<0x>3.
  由于在 x = 0 x=0 x=0两侧 f ′ ′ ( x ) f''(x) f(x)异号,因此 ( 0 , f ( 0 ) ) = ( 0 , 0 ) (0,f(0))=(0,0) (0,f(0))=(0,0)为曲线 y = f ( x ) y=f(x) y=f(x)的拐点。故选 ( C ) (C) (C)。(这道题主要利用了拐点和极值点定义求解

14.设函数 f ( x ) f(x) f(x)可导,且满足 x f ′ ( x ) = f ′ ( − x ) + 1 , f ( 0 ) = 0 xf'(x)=f'(-x)+1,f(0)=0 xf(x)=f(x)+1,f(0)=0,求:

(1) f ′ ( x ) f'(x) f(x)

  在方程 x f ′ ( x ) = f ′ ( − x ) + 1 xf'(x)=f'(-x)+1 xf(x)=f(x)+1中用 − x -x x代替 x x x,得 − x f ′ ( − x ) = f ′ ( x ) + 1 -xf'(-x)=f'(x)+1 xf(x)=f(x)+1,从而有
{ x f ′ ( x ) = f ′ ( − x ) + 1 , − x f ′ ( − x ) = f ′ ( x ) + 1. \begin{cases} xf'(x)=f'(-x)+1,\\ -xf'(-x)=f'(x)+1. \end{cases} { xf(x)=f(x)+1,xf(x)=f(x)+1.
  解得 f ′ ( x ) = x − 1 1 + x 2 f'(x)=\cfrac{x-1}{1+x^2} f(x)=1+x2x1

(2)函数 f ( x ) f(x) f(x)的极值。

  由 f ( 0 ) = 0 f(0)=0 f(0)=0,得 f ( x ) − f ( 0 ) = ∫ 0 x t − 1 1 + t 2 d t f(x)-f(0)=\displaystyle\int^x_0\cfrac{t-1}{1+t^2}\mathrm{d}t f(x)f(0)=0x1+t2t1dt,即 f ( x ) = 1 2 ln ⁡ ( 1 + x 2 ) − arctan ⁡ x f(x)=\cfrac{1}{2}\ln(1+x^2)-\arctan x f(x)=21ln(1+x2)arctanx
  由 f ′ ( x ) = x − 1 1 + x 2 f'(x)=\cfrac{x-1}{1+x^2} f(x)=1+x2x1,得函数 f ( x ) f(x) f(x)的驻点 x 0 = 1 x_0=1 x0=1,且唯一。而 f ′ ′ ( x ) = − x 2 + 2 x + 1 ( 1 + x 2 ) 2 f''(x)=\cfrac{-x^2+2x+1}{(1+x^2)^2} f(x)=(1+x2)2x2+2x+1,所以 f ′ ′ ( 1 ) > 0 f''(1)>0 f(1)>0。故 f ( 1 ) = 1 2 ln ⁡ 2 − π 4 f(1)=\cfrac{1}{2}\ln2-\cfrac{\pi}{4} f(1)=21ln24π是函数 f ( x ) f(x) f(x)的极小值。(这道题主要利用了构造方程求解

C C C

10.设 f ( x ) = { lim ⁡ n → ∞ 1 n ( 1 + cos ⁡ x n + cos ⁡ 2 x n + ⋯ + cos ⁡ n − 1 n x ) , x > 0 , 1 , x = 0 , f ( − x ) , x < 0. f(x)=\begin{cases}\lim\limits_{n\to\infty}\frac{1}{n}\left(1+\cos\frac{x}{n}+\cos\frac{2x}{n}+\cdots+\cos\frac{n-1}{n}x\right),&x>0,\\1,&x=0,\\f(-x),x<0.\end{cases} f(x)=nlimn1(1+cosnx+cosn2x++cosnn1x),1,f(x),x<0.x>0,x=0,

(1)求 f ′ ( 0 ) f'(0) f(0)

  当 x > 0 x>0 x>0时,
f ( x ) = lim ⁡ n → ∞ 1 n ∑ i = 0 n − 1 cos ⁡ i n x = lim ⁡ n → ∞ 1 x ∑ i = 0 n − 1 cos ⁡ i n x ⋅ x n = 1 x ∫ 0 1 cos ⁡ t d t = 1 x sin ⁡ t ∣ 0 x = sin ⁡ x x ; \begin{aligned} f(x)&=\lim\limits_{n\to\infty}\cfrac{1}{n}\sum\limits_{i=0}^{n-1}\cos\cfrac{i}{n}x=\lim\limits_{n\to\infty}\cfrac{1}{x}\sum\limits_{i=0}^{n-1}\cos\cfrac{i}{n}x\cdot\cfrac{x}{n}\\ &=\cfrac{1}{x}\displaystyle\int^1_0\cos t\mathrm{d}t=\cfrac{1}{x}\sin t\biggm\vert_0^x=\cfrac{\sin x}{x}; \end{aligned} f(x)=nlimn1i=0n1cosnix=nlimx1i=0n1cosnixnx=x101costdt=x1sint0x=xsinx;
  当 x < 0 x<0 x<0时, f ( − x ) = sin ⁡ ( − x ) − x = sin ⁡ x x f(-x)=\cfrac{\sin(-x)}{-x}=\cfrac{\sin x}{x} f(x)=xsin(x)=xsinx
  综上所述, f ( x ) = { sin ⁡ x x , x ≠ 0 , 1 , x = 0. f(x)=\begin{cases}\cfrac{\sin x}{x},&x\ne0,\\1,&x=0.\end{cases} f(x)=xsinx,1,x=0,x=0.
  故
f ′ ( 0 ) = lim ⁡ x → 0 f ( x ) − f ( 0 ) x − 0 = lim ⁡ x → 0 sin ⁡ x x − 1 x = lim ⁡ x → 0 sin ⁡ x − x x 2 = 0. \begin{aligned} f'(0)&=\lim\limits_{x\to0}\cfrac{f(x)-f(0)}{x-0}=\lim\limits_{x\to0}\cfrac{\cfrac{\sin x}{x}-1}{x}\\ &=\lim\limits_{x\to0}\cfrac{\sin x-x}{x^2}=0. \end{aligned} f(0)=x0limx0f(x)f(0)=x0limxxsinx1=x0limx2sinxx=0.

(2)求 f ( x ) f(x) f(x) [ − π , π ] [-\pi,\pi] [π,π]上的最大值。

  由 f ( x ) f(x) f(x) [ − π , π ] [-\pi,\pi] [π,π]上的偶函数,故只研究 [ 0 , π ] [0,\pi] [0,π]上的情形即可。
  当 0 < x ⩽ π 00<xπ时, f ′ ( x ) = x cos ⁡ x − sin ⁡ x x 2 f'(x)=\cfrac{x\cos x-\sin x}{x^2} f(x)=x2xcosxsinx,令 g ( x ) = x cos ⁡ x − sin ⁡ x g(x)=x\cos x-\sin x g(x)=xcosxsinx,则 g ′ ( x ) = − x sin ⁡ x ⩽ 0 g'(x)=-x\sin x\leqslant0 g(x)=xsinx0,且仅当 x = π x=\pi x=π时, g ′ ( x ) = 0 g'(x)=0 g(x)=0,故 g ( x ) g(x) g(x) ( 0 , π ] (0,\pi] (0,π]严格单调递减, g ( x ) < g ( 0 ) = 0 g(x)g(x)<g(0)=0,于是 f ′ ( x ) < 0 f'(x)<0 f(x)<0 f ( x ) f(x) f(x)单调递减,则 f ( x ) f(x) f(x)的最大值在 x = 0 x=0 x=0处取得, f max ⁡ = f ( 0 ) = 1 f_{\max}=f(0)=1 fmax=f(0)=1。(这道题主要利用了积分定义求解

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