张宇1000题高等数学 第六章 一元函数微分学的应用(二)——中值定理、微分等式与微分不等式

目录

  • A A A
    • 1.设函数 f ( x ) f(x) f(x)在区间 ( a , b ) (a,b) (a,b)内可导, x 1 x_1 x1 x 2 x_2 x2是区间 ( a , b ) (a,b) (a,b)内任意两点,且 x 1 < x 2 x_1x1<x2,则至少存在一点 ξ \xi ξ,使(  )
      ( A ) f ( b ) − f ( a ) = f ′ ( ξ ) ( b − a ) , (A)f(b)-f(a)=f'(\xi)(b-a), (A)f(b)f(a)=f(ξ)(ba),其中 a < ξ < b a<\xia<ξ<b
      ( B ) f ( b ) − f ( x 1 ) = f ′ ( ξ ) ( b − x 1 ) , (B)f(b)-f(x_1)=f'(\xi)(b-x_1), (B)f(b)f(x1)=f(ξ)(bx1),其中 x 1 < ξ < b x_1<\xix1<ξ<b
      ( C ) f ( x 2 ) − f ( x 1 ) = f ′ ( ξ ) ( x 2 − x 1 ) , (C)f(x_2)-f(x_1)=f'(\xi)(x_2-x_1), (C)f(x2)f(x1)=f(ξ)(x2x1),其中 x 1 < ξ < x 2 x_1<\xix1<ξ<x2
      ( D ) f ( x 2 ) − f ( a ) = f ′ ( ξ ) ( b − a ) , (D)f(x_2)-f(a)=f'(\xi)(b-a), (D)f(x2)f(a)=f(ξ)(ba),其中 a < ξ < x 2 a<\xia<ξ<x2
  • B B B
    • 4.设 f ( x ) f(x) f(x) [ 0 , 1 ] [0,1] [0,1]上可导, f ( 0 ) = 0 , ∣ f ′ ( x ) ∣ ⩽ ∣ f ( x ) ∣ f(0)=0,|f'(x)|\leqslant|f(x)| f(0)=0,f(x)f(x),则 f ( 1 ) = f(1)= f(1)=______。
    • 8.设函数 f ( x ) f(x) f(x) [ a , b ] [a,b] [a,b]上连续,在 ( a , b ) (a,b) (a,b)内可导且 f ( a ) ≠ f ( b ) f(a)\ne f(b) f(a)=f(b),证明:存在 ξ , η ∈ ( a , b ) \xi,\eta\in(a,b) ξ,η(a,b),使得 f ′ ( ξ ) 2 ξ = f ′ ( η ) b + a \cfrac{f'(\xi)}{2\xi}=\cfrac{f'(\eta)}{b+a} 2ξf(ξ)=b+af(η)
    • 9.设函数 f ( x ) f(x) f(x) [ − 2 , 2 ] [-2,2] [2,2]上二阶可导,且 ∣ f ( x ) ∣ ⩽ 1 |f(x)|\leqslant1 f(x)1,又 f 2 ( 0 ) + [ f ′ ( 0 ) ] 2 = 4 f^2(0)+[f'(0)]^2=4 f2(0)+[f(0)]2=4。证明:在 ( − 2 , 2 ) (-2,2) (2,2)内至少存在一点 ξ \xi ξ,使得 f ( ξ ) + f ′ ′ ( ξ ) = 0 f(\xi)+f''(\xi)=0 f(ξ)+f(ξ)=0
    • 11.设 f ( x ) f(x) f(x)在闭区间 [ 1 , 2 ] [1,2] [1,2]上可导,证明:存在一点 ξ ∈ ( 1 , 2 ) \xi\in(1,2) ξ(1,2),使 f ( 2 ) − 2 f ( 1 ) = ξ f ′ ( ξ ) − f ( ξ ) f(2)-2f(1)=\xi f'(\xi)-f(\xi) f(2)2f(1)=ξf(ξ)f(ξ)
    • 12.设 f ( x ) f(x) f(x)在闭区间 [ − 1 , 1 ] [-1,1] [1,1]上有三阶连续导数,且 f ( − 1 ) = 0 , f ( 1 ) = 1 , f ′ ( 0 ) = 0 f(-1)=0,f(1)=1,f'(0)=0 f(1)=0,f(1)=1,f(0)=0。证明:在 [ − 1 , 1 ] [-1,1] [1,1]内存在一点 ξ \xi ξ,使得 f ′ ′ ′ ( ξ ) = 3 f'''(\xi)=3 f(ξ)=3
    • 22.证明: e x + e − x ⩾ 2 x 2 + 2 cos ⁡ x , − ∞ < x < + ∞ e^x+e^{-x}\geqslant2x^2+2\cos x,-\inftyex+ex2x2+2cosx,<x<+
    • 23.设 f ( x ) f(x) f(x)在区间 [ 0 , + ∞ ) [0,+\infty) [0,+)内有二阶导数,且 ∣ f ( x ) ∣ ⩽ 1 , 0 < ∣ f ′ ′ ( x ) ∣ ⩽ 2 ( 0 ⩽ x < + ∞ ) |f(x)|\leqslant1,0<|f''(x)|\leqslant2(0\leqslant x<+\infty) f(x)1,0<f(x)2(0x<+)。证明: ∣ f ′ ( x ) ∣ ⩽ 2 2 |f'(x)|\leqslant2\sqrt{2} f(x)22
  • C C C
    • 3.设 f ( x ) , g ( x ) f(x),g(x) f(x),g(x) [ a , b ] [a,b] [a,b]上二阶可导,且 f ( a ) = f ( b ) = g ( a ) = 0 f(a)=f(b)=g(a)=0 f(a)=f(b)=g(a)=0,证明:存在 ξ ∈ ( a , b ) \xi\in(a,b) ξ(a,b),使 f ′ ′ ( ξ ) g ( ξ ) + 2 f ′ ( ξ ) g ′ ( ξ ) + f ( ξ ) g ′ ′ ( ξ ) = 0 f''(\xi)g(\xi)+2f'(\xi)g'(\xi)+f(\xi)g''(\xi)=0 f(ξ)g(ξ)+2f(ξ)g(ξ)+f(ξ)g(ξ)=0
    • 6.设 f ( x ) f(x) f(x) [ a , b ] [a,b] [a,b]上有二阶导数,且 f ′ ′ ( x ) > 0 f''(x)>0 f(x)>0,证明: f ( a + b 2 ) < 1 b − a ∫ a b f ( t ) d t < 1 2 [ f ( a ) + f ( b ) ] f\left(\cfrac{a+b}{2}\right)<\cfrac{1}{b-a}\displaystyle\int^b_af(t)\mathrm{d}t<\cfrac{1}{2}[f(a)+f(b)] f(2a+b)<ba1abf(t)dt<21[f(a)+f(b)]
  • 写在最后

A A A

1.设函数 f ( x ) f(x) f(x)在区间 ( a , b ) (a,b) (a,b)内可导, x 1 x_1 x1 x 2 x_2 x2是区间 ( a , b ) (a,b) (a,b)内任意两点,且 x 1 < x 2 x_1x1<x2,则至少存在一点 ξ \xi ξ,使(  )
( A ) f ( b ) − f ( a ) = f ′ ( ξ ) ( b − a ) , (A)f(b)-f(a)=f'(\xi)(b-a), (A)f(b)f(a)=f(ξ)(ba),其中 a < ξ < b a<\xia<ξ<b
( B ) f ( b ) − f ( x 1 ) = f ′ ( ξ ) ( b − x 1 ) , (B)f(b)-f(x_1)=f'(\xi)(b-x_1), (B)f(b)f(x1)=f(ξ)(bx1),其中 x 1 < ξ < b x_1<\xix1<ξ<b
( C ) f ( x 2 ) − f ( x 1 ) = f ′ ( ξ ) ( x 2 − x 1 ) , (C)f(x_2)-f(x_1)=f'(\xi)(x_2-x_1), (C)f(x2)f(x1)=f(ξ)(x2x1),其中 x 1 < ξ < x 2 x_1<\xix1<ξ<x2
( D ) f ( x 2 ) − f ( a ) = f ′ ( ξ ) ( b − a ) , (D)f(x_2)-f(a)=f'(\xi)(b-a), (D)f(x2)f(a)=f(ξ)(ba),其中 a < ξ < x 2 a<\xia<ξ<x2

  由于拉格朗日中值定理需要函数 f ( x ) f(x) f(x)在闭区间 [ a , b ] [a,b] [a,b]上连续,在开区间 ( a , b ) (a,b) (a,b)上可导。而题设条件只给出 f ( x ) f(x) f(x) ( a , b ) (a,b) (a,b)内可导,仅可知 f ( x ) f(x) f(x) ( a , b ) (a,b) (a,b)上连续。故选 ( C ) (C) (C)。(这道题主要利用了拉格朗日中值定理使用条件求解

B B B

4.设 f ( x ) f(x) f(x) [ 0 , 1 ] [0,1] [0,1]上可导, f ( 0 ) = 0 , ∣ f ′ ( x ) ∣ ⩽ ∣ f ( x ) ∣ f(0)=0,|f'(x)|\leqslant|f(x)| f(0)=0,f(x)f(x),则 f ( 1 ) = f(1)= f(1)=______。

  当 0 < x < 1 00<x<1时,
∣ f ( x ) ∣ = ∣ f ( x ) − f ( 0 ) ∣ = ∣ f ′ ( ξ 1 ) ∣ x ⩽ ∣ f ( ξ 1 ) ∣ ⋅ x = ∣ f ( ξ 1 ) − f ( 0 ) ∣ ⋅ x = ∣ f ′ ( ξ 2 ) ∣ ⋅ ξ 2 ⋅ x ⩽ ∣ f ( ξ 2 ) ∣ ⋅ x 2 ⩽ ⋯ ⩽ ∣ f ( ξ n ) ∣ ⋅ x n ⩽ M ⋅ x n → n → ∞ 0 , \begin{aligned} |f(x)|&=|f(x)-f(0)|=|f'(\xi_1)|x\leqslant|f(\xi_1)|\cdot x\\ &=|f(\xi_1)-f(0)|\cdot x=|f'(\xi_2)|\cdot\xi_2\cdot x\\ &\leqslant|f(\xi_2)|\cdot x^2\leqslant\cdots\leqslant|f(\xi_n)|\cdot x^n\\ &\leqslant M\cdot x^n\xrightarrow[]{n\to\infty}0, \end{aligned} f(x)=f(x)f(0)=f(ξ1)xf(ξ1)x=f(ξ1)f(0)x=f(ξ2)ξ2xf(ξ2)x2f(ξn)xnMxnn 0,
  其中 0 < ξ n < ⋯ < ξ 2 < ξ 1 < x 0<\xi_n<\cdots<\xi_2<\xi_10<ξn<<ξ2<ξ1<x M M M ∣ f ( x ) ∣ |f(x)| f(x) [ 0 , 1 ] [0,1] [0,1]上的最大值,故当 0 < x < 1 00<x<1时, f ( x ) = 0 f(x)=0 f(x)=0,又由 f ( x ) f(x) f(x) x = 1 x=1 x=1处左连续,有 f ( 1 ) = lim ⁡ x → 1 − f ( x ) = 0 f(1)=\lim\limits_{x\to1^-}f(x)=0 f(1)=x1limf(x)=0。(这道题主要利用了放缩法求解

8.设函数 f ( x ) f(x) f(x) [ a , b ] [a,b] [a,b]上连续,在 ( a , b ) (a,b) (a,b)内可导且 f ( a ) ≠ f ( b ) f(a)\ne f(b) f(a)=f(b),证明:存在 ξ , η ∈ ( a , b ) \xi,\eta\in(a,b) ξ,η(a,b),使得 f ′ ( ξ ) 2 ξ = f ′ ( η ) b + a \cfrac{f'(\xi)}{2\xi}=\cfrac{f'(\eta)}{b+a} 2ξf(ξ)=b+af(η)

  对 f ( x ) f(x) f(x)应用拉格朗日中值定理知 f ( b ) − f ( a ) = f ′ ( η ) ( b − a ) , η ∈ ( a , b ) f(b)-f(a)=f'(\eta)(b-a),\eta\in(a,b) f(b)f(a)=f(η)(ba),η(a,b),对 f ( x ) , x 2 f(x),x^2 f(x),x2 [ a , b ] [a,b] [a,b]上应用柯西中值定理知 f ( b ) − f ( a ) b 2 − a 2 = f ′ ( ξ ) 2 ξ , ξ ∈ ( a , b ) \cfrac{f(b)-f(a)}{b^2-a^2}=\cfrac{f'(\xi)}{2\xi},\xi\in(a,b) b2a2f(b)f(a)=2ξf(ξ),ξ(a,b)。所以 f ( b ) − f ( a ) = f ′ ( ξ ) 2 ξ ( b 2 − a 2 ) f(b)-f(a)=\cfrac{f'(\xi)}{2\xi}(b^2-a^2) f(b)f(a)=2ξf(ξ)(b2a2),则 f ′ ( η ) ( b − a ) = f ′ ( ξ ) 2 ξ ( b 2 − a 2 ) f'(\eta)(b-a)=\cfrac{f'(\xi)}{2\xi}(b^2-a^2) f(η)(ba)=2ξf(ξ)(b2a2),即 f ′ ( ξ ) 2 ξ = f ′ ( η ) b + a \cfrac{f'(\xi)}{2\xi}=\cfrac{f'(\eta)}{b+a} 2ξf(ξ)=b+af(η)。(这道题主要利用了联合使用定理求解

9.设函数 f ( x ) f(x) f(x) [ − 2 , 2 ] [-2,2] [2,2]上二阶可导,且 ∣ f ( x ) ∣ ⩽ 1 |f(x)|\leqslant1 f(x)1,又 f 2 ( 0 ) + [ f ′ ( 0 ) ] 2 = 4 f^2(0)+[f'(0)]^2=4 f2(0)+[f(0)]2=4。证明:在 ( − 2 , 2 ) (-2,2) (2,2)内至少存在一点 ξ \xi ξ,使得 f ( ξ ) + f ′ ′ ( ξ ) = 0 f(\xi)+f''(\xi)=0 f(ξ)+f(ξ)=0

  由拉格朗日中值定理有
f ( 0 ) − f ( − 2 ) = 2 f ′ ( ξ 1 ) , − 2 < ξ 1 < 0 , f ( 2 ) − f ( 0 ) = 2 f ′ ( ξ 2 ) , 0 < ξ 2 < 2. f(0)-f(-2)=2f'(\xi_1),-2<\xi_1<0,\\ f(2)-f(0)=2f'(\xi_2),0<\xi_2<2. f(0)f(2)=2f(ξ1),2<ξ1<0,f(2)f(0)=2f(ξ2),0<ξ2<2.
  由 ∣ f ( x ) ∣ ⩽ 1 |f(x)|\leqslant1 f(x)1 ∣ f ′ ( ξ 1 ) ∣ = ∣ f ( 0 ) − f ( − 2 ) ∣ 2 ⩽ 1 ; ∣ f ′ ( ξ 2 ) ∣ = ∣ f ( 2 ) − f ( 0 ) ∣ 2 ⩽ 1 |f'(\xi_1)|=\cfrac{|f(0)-f(-2)|}{2}\leqslant1;|f'(\xi_2)|=\cfrac{|f(2)-f(0)|}{2}\leqslant1 f(ξ1)=2f(0)f(2)1;f(ξ2)=2f(2)f(0)1
  令 φ ( x ) = f 2 ( x ) + [ f ′ ( x ) ] 2 \varphi(x)=f^2(x)+[f'(x)]^2 φ(x)=f2(x)+[f(x)]2,则有 φ ( ξ 1 ) ⩽ 2 , φ ( ξ 2 ) ⩽ 2 \varphi(\xi_1)\leqslant2,\varphi(\xi_2)\leqslant2 φ(ξ1)2,φ(ξ2)2
  因为 φ ( x ) \varphi(x) φ(x) [ ξ 1 , ξ 2 ] [\xi_1,\xi_2] [ξ1,ξ2]上连续,且 φ ( 0 ) = 4 \varphi(0)=4 φ(0)=4,设 φ ( x ) \varphi(x) φ(x) [ ξ 1 , ξ 2 ] [\xi_1,\xi_2] [ξ1,ξ2]上的最大值在 ξ ∈ ( ξ 1 , ξ 2 ) ⊂ ( − 2 , 2 ) \xi\in(\xi_1,\xi_2)\subset(-2,2) ξ(ξ1,ξ2)(2,2)处取到,则 φ ( x ) ⩽ 4 \varphi(x)\leqslant4 φ(x)4,且 φ ( x ) \varphi(x) φ(x) [ ξ 1 , ξ 2 ] [\xi_1,\xi_2] [ξ1,ξ2]上可导,由费马定理有 φ ′ ( x ) = 0 \varphi'(x)=0 φ(x)=0,即 2 f ( ξ ) ⋅ f ′ ( ξ ) + f ′ ( ξ ) ⋅ f ′ ′ ( ξ ) = 0 2f(\xi)\cdot f'(\xi)+f'(\xi)\cdot f''(\xi)=0 2f(ξ)f(ξ)+f(ξ)f(ξ)=0
  因为 ∣ f ( x ) ∣ ⩽ 1 |f(x)|\leqslant1 f(x)1,且 φ ( x ) ⩽ 4 \varphi(x)\leqslant4 φ(x)4,所以 f ′ ( ξ ) ≠ 0 f'(\xi)\ne0 f(ξ)=0,于是有 f ( ξ ) + f ′ ′ ( ξ ) = 0 , ξ ∈ ( − 2 , 2 ) f(\xi)+f''(\xi)=0,\xi\in(-2,2) f(ξ)+f(ξ)=0,ξ(2,2)。(这道题主要利用了构造函数求解

11.设 f ( x ) f(x) f(x)在闭区间 [ 1 , 2 ] [1,2] [1,2]上可导,证明:存在一点 ξ ∈ ( 1 , 2 ) \xi\in(1,2) ξ(1,2),使 f ( 2 ) − 2 f ( 1 ) = ξ f ′ ( ξ ) − f ( ξ ) f(2)-2f(1)=\xi f'(\xi)-f(\xi) f(2)2f(1)=ξf(ξ)f(ξ)

  令 F ( x ) = f ( x ) + f ( 2 ) − 2 f ( 1 ) x F(x)=\cfrac{f(x)+f(2)-2f(1)}{x} F(x)=xf(x)+f(2)2f(1) F ( x ) F(x) F(x) [ 1 , 2 ] [1,2] [1,2]上连续,在 ( 1 , 2 ) (1,2) (1,2)内可导,且 F ( 2 ) = F ( 1 ) = f ( 2 ) − f ( 1 ) F(2)=F(1)=f(2)-f(1) F(2)=F(1)=f(2)f(1)。由罗尔定理,存在 ξ ∈ ( 1 , 2 ) \xi\in(1,2) ξ(1,2),使 F ′ ( ξ ) = 0 F'(\xi)=0 F(ξ)=0,即 f ( 2 ) − 2 f ( 1 ) = ξ f ′ ( ξ ) − f ( ξ ) f(2)-2f(1)=\xi f'(\xi)-f(\xi) f(2)2f(1)=ξf(ξ)f(ξ)。(这道题主要利用了构造函数求解

12.设 f ( x ) f(x) f(x)在闭区间 [ − 1 , 1 ] [-1,1] [1,1]上有三阶连续导数,且 f ( − 1 ) = 0 , f ( 1 ) = 1 , f ′ ( 0 ) = 0 f(-1)=0,f(1)=1,f'(0)=0 f(1)=0,f(1)=1,f(0)=0。证明:在 [ − 1 , 1 ] [-1,1] [1,1]内存在一点 ξ \xi ξ,使得 f ′ ′ ′ ( ξ ) = 3 f'''(\xi)=3 f(ξ)=3


f ( x ) = f ( x 0 ) + f ′ ( x 0 ) ( x − x 0 ) + 1 2 ! f ′ ′ ( x 0 ) ( x − x 0 ) 2 + 1 3 ! f ′ ′ ′ ( η ) ( x − x 0 ) 3 . f(x)=f(x_0)+f'(x_0)(x-x_0)+\cfrac{1}{2!}f''(x_0)(x-x_0)^2+\cfrac{1}{3!}f'''(\eta)(x-x_0)^3. f(x)=f(x0)+f(x0)(xx0)+2!1f(x0)(xx0)2+3!1f(η)(xx0)3.
  取 x 0 = 0 , x = 1 x_0=0,x=1 x0=0,x=1代入,得
f ( 1 ) = f ( 0 ) + f ′ ( 0 ) ( 1 − 0 ) + 1 2 f ′ ′ ( 0 ) ( 1 − 0 ) 2 + 1 6 f ′ ′ ′ ( η 1 ) ( 1 − 0 ) 3 , η 1 ∈ ( 0 , 1 ) . (1) f(1)=f(0)+f'(0)(1-0)+\cfrac{1}{2}f''(0)(1-0)^2+\cfrac{1}{6}f'''(\eta_1)(1-0)^3,\eta_1\in(0,1).\tag{1} f(1)=f(0)+f(0)(10)+21f(0)(10)2+61f(η1)(10)3,η1(0,1).(1)
  取 x 0 = 0 , x = − 1 x_0=0,x=-1 x0=0,x=1代入,得
f ( − 1 ) = f ( 0 ) + f ′ ( 0 ) ( − 1 − 0 ) + 1 2 f ′ ′ ( 0 ) ( − 1 − 0 ) 2 + 1 6 f ′ ′ ′ ( η 2 ) ( − 1 − 0 ) 3 , η 2 ∈ ( 0 , 1 ) . (2) f(-1)=f(0)+f'(0)(-1-0)+\cfrac{1}{2}f''(0)(-1-0)^2+\cfrac{1}{6}f'''(\eta_2)(-1-0)^3,\eta_2\in(0,1).\tag{2} f(1)=f(0)+f(0)(10)+21f(0)(10)2+61f(η2)(10)3,η2(0,1).(2)
   ( 1 ) − ( 2 ) (1)-(2) (1)(2)
f ( 1 ) − f ( − 1 ) = 1 6 [ f ′ ′ ′ ( η 1 ) + f ′ ′ ′ ( η 2 ) ] = 1. (3) f(1)-f(-1)=\cfrac{1}{6}[f'''(\eta_1)+f'''(\eta_2)]=1.\tag{3} f(1)f(1)=61[f(η1)+f(η2)]=1.(3)
  因为 f ′ ′ ′ ( x ) f'''(x) f(x) [ − 1 , 1 ] [-1,1] [1,1]上连续,则存在 m m m M M M,使得 ∀ x ∈ [ − 1 , 1 ] , m ⩽ f ′ ′ ′ ( x ) ⩽ M \forall x\in[-1,1],m\leqslant f'''(x)\leqslant M x[1,1],mf(x)M
m ⩽ f ′ ′ ′ ( η 1 ) ⩽ M , m ⩽ f ′ ′ ′ ( η 2 ) ⩽ M ⇒ 1 2 [ f ′ ′ ′ ( η 1 ) + f ′ ′ ′ ( η 2 ) ] ⩽ M . (4) m\leqslant f'''(\eta_1)\leqslant M,m\leqslant f'''(\eta_2)\leqslant M\\ \Rightarrow\cfrac{1}{2}[f'''(\eta_1)+f'''(\eta_2)]\leqslant M.\tag{4} mf(η1)M,mf(η2)M21[f(η1)+f(η2)]M.(4)
   ( 3 ) (3) (3)代入 ( 4 ) (4) (4),有 m ⩽ 3 ⩽ M m\leqslant3\leqslant M m3M,由介值定理,存在 ξ ∈ [ − 1 , 1 ] \xi\in[-1,1] ξ[1,1],使得 f ′ ′ ′ ( ξ ) = 3 f'''(\xi)=3 f(ξ)=3。(这道题主要利用了介值定理求解

22.证明: e x + e − x ⩾ 2 x 2 + 2 cos ⁡ x , − ∞ < x < + ∞ e^x+e^{-x}\geqslant2x^2+2\cos x,-\inftyex+ex2x2+2cosx,<x<+

  令 f ( x ) = e x + e − x − 2 x 2 − 2 cos ⁡ x f(x)=e^x+e^{-x}-2x^2-2\cos x f(x)=ex+ex2x22cosx。显然 f ( x ) f(x) f(x)为偶函数, f ( 0 ) = 0 f(0)=0 f(0)=0。只需证 x ⩾ 0 x\geqslant0 x0的情形。由
f ′ ( x ) = e x − e − x − 4 x + 2 sin ⁡ x , f ′ ′ ( x ) = e x + e − x − 4 + 2 cos ⁡ x , f ′ ′ ′ ( x ) = e x − e − x − 2 sin ⁡ x , f ( 4 ) ( x ) = e x + e − x − 2 cos ⁡ x , f'(x)=e^x-e^{-x}-4x+2\sin x,f''(x)=e^x+e^{-x}-4+2\cos x,\\ f'''(x)=e^x-e^{-x}-2\sin x,f^{(4)}(x)=e^x+e^{-x}-2\cos x, f(x)=exex4x+2sinx,f(x)=ex+ex4+2cosx,f(x)=exex2sinx,f(4)(x)=ex+ex2cosx,
  知当 x > 0 x>0 x>0时, f ( 4 ) ( x ) > 2 e x ⋅ e − x − 2 cos ⁡ x = 2 ( 1 − cos ⁡ x ) f^{(4)}(x)>2\sqrt{e^x\cdot e^{-x}}-2\cos x=2(1-\cos x) f(4)(x)>2exex 2cosx=2(1cosx),且 f ( 0 ) = f ′ ( 0 ) = f ′ ′ ( 0 ) = f ′ ′ ′ ( 0 ) = f ( 4 ) ( 0 ) = 0 f(0)=f'(0)=f''(0)=f'''(0)=f^{(4)}(0)=0 f(0)=f(0)=f(0)=f(0)=f(4)(0)=0,所以 f ( x ) = f ( 0 ) + f ′ ( 0 ) x + f ′ ′ ( 0 ) 2 ! x 2 + f ′ ′ ′ ( 0 ) 3 ! x 3 + f ( 4 ) ( ξ ) 4 ! x 4 = f ( 4 ) ( ξ ) 4 ! x 4 > 0 ( ξ 介于 0 与 x 之间 ) f(x)=f(0)+f'(0)x+\cfrac{f''(0)}{2!}x^2+\cfrac{f'''(0)}{3!}x^3+\cfrac{f^{(4)}(\xi)}{4!}x^4=\cfrac{f^{(4)}(\xi)}{4!}x^4>0(\xi\text{介于}0\text{与}x\text{之间}) f(x)=f(0)+f(0)x+2!f(0)x2+3!f(0)x3+4!f(4)(ξ)x4=4!f(4)(ξ)x4>0(ξ介于0x之间),即当 x ⩾ 0 x\geqslant0 x0时, f ( x ) ⩾ 0 f(x)\geqslant0 f(x)0。故 e x + e − x ⩾ 2 x 2 + 2 cos ⁡ x , − ∞ < x < + ∞ e^x+e^{-x}\geqslant2x^2+2\cos x,-\inftyex+ex2x2+2cosx,<x<+。(这道题主要利用了泰勒展开式求解

23.设 f ( x ) f(x) f(x)在区间 [ 0 , + ∞ ) [0,+\infty) [0,+)内有二阶导数,且 ∣ f ( x ) ∣ ⩽ 1 , 0 < ∣ f ′ ′ ( x ) ∣ ⩽ 2 ( 0 ⩽ x < + ∞ ) |f(x)|\leqslant1,0<|f''(x)|\leqslant2(0\leqslant x<+\infty) f(x)1,0<f(x)2(0x<+)。证明: ∣ f ′ ( x ) ∣ ⩽ 2 2 |f'(x)|\leqslant2\sqrt{2} f(x)22

  对任意的 x ∈ [ 0 , + ∞ ) x\in[0,+\infty) x[0,+),及任意的 h > 0 h>0 h>0,使 x + h ∈ ( 0 , + ∞ ) x+h\in(0,+\infty) x+h(0,+),于是有 f ( x + h ) = f ( x ) + f ′ ( x ) h + 1 2 ! f ′ ′ ( ξ ) h 2 f(x+h)=f(x)+f'(x)h+\cfrac{1}{2!}f''(\xi)h^2 f(x+h)=f(x)+f(x)h+2!1f(ξ)h2,其中 ξ ∈ ( x , x + h ) \xi\in(x,x+h) ξ(x,x+h),即 f ′ ( x ) = 1 h [ f ( x + h ) − f ( x ) ] − h 2 f ′ ′ ( ξ ) f'(x)=\cfrac{1}{h}[f(x+h)-f(x)]-\cfrac{h}{2}f''(\xi) f(x)=h1[f(x+h)f(x)]2hf(ξ),故 ∣ f ′ ( x ) ∣ ⩽ 1 h [ ∣ f ( x + h ) ∣ + ∣ f ( x ) ∣ ] + h 2 ∣ f ′ ′ ( ξ ) ∣ ⩽ 2 h + h |f'(x)|\leqslant\cfrac{1}{h}[|f(x+h)|+|f(x)|]+\cfrac{h}{2}|f''(\xi)|\leqslant\cfrac{2}{h}+h f(x)h1[f(x+h)+f(x)]+2hf(ξ)h2+h
  令 g ( h ) = 2 h + h ( h > 0 ) g(h)=\cfrac{2}{h}+h(h>0) g(h)=h2+h(h>0),求其最小值。
  令 g ′ ( h ) = − 2 h 2 + 1 = 0 g'(h)=-\cfrac{2}{h^2}+1=0 g(h)=h22+1=0,得到驻点 h 0 = 2 , g ′ ′ ( h ) = 4 h 3 > 0 h_0=\sqrt{2},g''(h)=\cfrac{4}{h^3}>0 h0=2 ,g(h)=h34>0,所以, g ( h ) g(h) g(h) h 0 = 2 h_0=\sqrt{2} h0=2 处取得极小值,即最小值 g ( h 0 ) = 2 2 g(h_0)=2\sqrt{2} g(h0)=22 。故 ∣ f ′ ( x ) ∣ ⩽ 2 2 |f'(x)|\leqslant2\sqrt{2} f(x)22 。(这道题主要利用了泰勒展开式求解

C C C

3.设 f ( x ) , g ( x ) f(x),g(x) f(x),g(x) [ a , b ] [a,b] [a,b]上二阶可导,且 f ( a ) = f ( b ) = g ( a ) = 0 f(a)=f(b)=g(a)=0 f(a)=f(b)=g(a)=0,证明:存在 ξ ∈ ( a , b ) \xi\in(a,b) ξ(a,b),使 f ′ ′ ( ξ ) g ( ξ ) + 2 f ′ ( ξ ) g ′ ( ξ ) + f ( ξ ) g ′ ′ ( ξ ) = 0 f''(\xi)g(\xi)+2f'(\xi)g'(\xi)+f(\xi)g''(\xi)=0 f(ξ)g(ξ)+2f(ξ)g(ξ)+f(ξ)g(ξ)=0

  令 F ( x ) = f ( x ) g ( x ) F(x)=f(x)g(x) F(x)=f(x)g(x),在 x = a x=a x=a处展开为泰勒公式,有
F ( x ) = F ( a ) + F ′ ( a ) ( x − a ) + 1 2 F ′ ′ ( ξ ) ( x − a ) 2 ( a < ξ < x ) , (1) F(x)=F(a)+F'(a)(x-a)+\cfrac{1}{2}F''(\xi)(x-a)^2(a<\xiF(x)=F(a)+F(a)(xa)+21F(ξ)(xa)2(a<ξ<x),(1)
  令 x = b x=b x=b,代入 ( 1 ) (1) (1)式,则
F ( b ) = F ( a ) + F ′ ( a ) ( b − a ) + 1 2 F ′ ′ ( ξ ) ( b − a ) 2 ( a < ξ < b ) . (2) F(b)=F(a)+F'(a)(b-a)+\cfrac{1}{2}F''(\xi)(b-a)^2(a<\xiF(b)=F(a)+F(a)(ba)+21F(ξ)(ba)2(a<ξ<b).(2)
  因 f ( a ) = f ( b ) = g ( a ) = 0 f(a)=f(b)=g(a)=0 f(a)=f(b)=g(a)=0,则 F ( a ) = F ( b ) = 0 F(a)=F(b)=0 F(a)=F(b)=0,且 F ′ ( a ) = 0 F'(a)=0 F(a)=0,代入 ( 2 ) (2) (2)式,得 F ′ ′ ( ξ ) = 0 F''(\xi)=0 F(ξ)=0,即 f ′ ′ ( ξ ) g ( ξ ) + 2 f ′ ( ξ ) g ′ ( ξ ) + f ( ξ ) g ′ ′ ( ξ ) = 0 f''(\xi)g(\xi)+2f'(\xi)g'(\xi)+f(\xi)g''(\xi)=0 f(ξ)g(ξ)+2f(ξ)g(ξ)+f(ξ)g(ξ)=0。(这道题主要利用了构造函数求解

6.设 f ( x ) f(x) f(x) [ a , b ] [a,b] [a,b]上有二阶导数,且 f ′ ′ ( x ) > 0 f''(x)>0 f(x)>0,证明: f ( a + b 2 ) < 1 b − a ∫ a b f ( t ) d t < 1 2 [ f ( a ) + f ( b ) ] f\left(\cfrac{a+b}{2}\right)<\cfrac{1}{b-a}\displaystyle\int^b_af(t)\mathrm{d}t<\cfrac{1}{2}[f(a)+f(b)] f(2a+b)<ba1abf(t)dt<21[f(a)+f(b)]

  先证左边。令 φ ( x ) = ( x − a ) f ( a + x 2 ) − ∫ a x f ( t ) d t \varphi(x)=(x-a)f\left(\cfrac{a+x}{2}\right)-\displaystyle\int^x_af(t)\mathrm{d}t φ(x)=(xa)f(2a+x)axf(t)dt,有 φ ( a ) = 0 \varphi(a)=0 φ(a)=0
φ ′ ( x ) = f ( a + x 2 ) + 1 2 ( x − a ) f ′ ( a + x 2 ) − f ( x ) = 1 2 ( x − a ) f ′ ( a + x 2 ) − [ f ( x ) − f ( a + x 2 ) ] = 1 2 [ f ′ ( a + x 2 ) − f ′ ( ξ ) ] , \begin{aligned} \varphi'(x)&=f\left(\cfrac{a+x}{2}\right)+\cfrac{1}{2}(x-a)f'\left(\cfrac{a+x}{2}\right)-f(x)\\ &=\cfrac{1}{2}(x-a)f'\left(\cfrac{a+x}{2}\right)-\left[f(x)-f\left(\cfrac{a+x}{2}\right)\right]\\ &=\cfrac{1}{2}\left[f'\left(\cfrac{a+x}{2}\right)-f'(\xi)\right], \end{aligned} φ(x)=f(2a+x)+21(xa)f(2a+x)f(x)=21(xa)f(2a+x)[f(x)f(2a+x)]=21[f(2a+x)f(ξ)],
  其中 a + x 2 < ξ < x \cfrac{a+x}{2}<\xi2a+x<ξ<x。由于 f ′ ′ ( x ) > 0 f''(x)>0 f(x)>0,所以 f ′ ( x ) f'(x) f(x)严格单调增加,从而 f ′ ( a + x 2 ) < f ′ ( ξ ) f'\left(\cfrac{a+x}{2}\right)f(2a+x)<f(ξ),于是 φ ′ ( x ) < 0 \varphi'(x)<0 φ(x)<0,所以当 x > a x>a x>a φ ( x ) < 0 \varphi(x)<0 φ(x)<0,有 φ ( b ) < 0 \varphi(b)<0 φ(b)<0
  再证右边。令 ψ ( x ) = ∫ a x f ( t ) d t − 1 2 ( x − a ) [ f ( a ) + f ( x ) ] \psi(x)=\displaystyle\int^x_af(t)\mathrm{d}t-\cfrac{1}{2}(x-a)[f(a)+f(x)] ψ(x)=axf(t)dt21(xa)[f(a)+f(x)],有 ψ ( a ) = 0 \psi(a)=0 ψ(a)=0
ψ ′ ( x ) = f ( x ) − 1 2 [ f ( a ) + f ( x ) ] − 1 2 ( x − a ) f ′ ( x ) = 1 2 [ f ( x ) − f ( a ) ] − 1 2 ( x − a ) f ′ ( x ) = 1 2 ( x − a ) [ f ′ ( η ) − f ′ ( x ) ] , \begin{aligned} \psi'(x)&=f(x)-\cfrac{1}{2}[f(a)+f(x)]-\cfrac{1}{2}(x-a)f'(x)\\ &=\cfrac{1}{2}[f(x)-f(a)]-\cfrac{1}{2}(x-a)f'(x)=\cfrac{1}{2}(x-a)[f'(\eta)-f'(x)], \end{aligned} ψ(x)=f(x)21[f(a)+f(x)]21(xa)f(x)=21[f(x)f(a)]21(xa)f(x)=21(xa)[f(η)f(x)],
  其中 a < η < x a<\etaa<η<x。由于 f ′ ′ ( x ) > 0 f''(x)>0 f(x)>0,所以 f ′ ( η ) < f ′ ( x ) f'(\eta)f(η)<f(x),从而 ψ ′ ( x ) < 0 \psi'(x)<0 ψ(x)<0。于是当 x > a x>a x>a时, ψ ( x ) < 0 \psi(x)<0 ψ(x)<0,故 ψ ( b ) < 0 \psi(b)<0 ψ(b)<0。(这道题主要利用了函数单调性求解

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