解 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)在 ( a , b ) (a,b) (a,b)内可导,仅可知 f ( x ) f(x) f(x)在 ( 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)
解 当 0 < x < 1 0
∣ 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)∣x⩽∣f(ξ1)∣⋅x=∣f(ξ1)−f(0)∣⋅x=∣f′(ξ2)∣⋅ξ2⋅x⩽∣f(ξ2)∣⋅x2⩽⋯⩽∣f(ξn)∣⋅xn⩽M⋅xnn→∞0,
其中 0 < ξ n < ⋯ < ξ 2 < ξ 1 < x 0<\xi_n<\cdots<\xi_2<\xi_1
解 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)(b−a),η∈(a,b),对 f ( x ) , x 2 f(x),x^2 f(x),x2在 [ 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) b2−a2f(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)(b2−a2),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)(b−a)=2ξf′(ξ)(b2−a2),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)
解 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知 ∣ 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)∣=2∣f(0)−f(−2)∣⩽1;∣f′(ξ2)∣=2∣f(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)在 [ 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)在 [ 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)在 [ 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)
解 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)在 [ 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)
解
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)(x−x0)+2!1f′′(x0)(x−x0)2+3!1f′′′(η)(x−x0)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)(1−0)+21f′′(0)(1−0)2+61f′′′(η1)(1−0)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)(−1−0)+21f′′(0)(−1−0)2+61f′′′(η2)(−1−0)3,η2∈(0,1).(2)
( 1 ) − ( 2 ) (1)-(2) (1)−(2)得
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)在 [ − 1 , 1 ] [-1,1] [−1,1]上连续,则存在 m m m和 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],m⩽f′′′(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} m⩽f′′′(η1)⩽M,m⩽f′′′(η2)⩽M⇒21[f′′′(η1)+f′′′(η2)]⩽M.(4)
( 3 ) (3) (3)代入 ( 4 ) (4) (4),有 m ⩽ 3 ⩽ M m\leqslant3\leqslant M m⩽3⩽M,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)
解 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+e−x−2x2−2cosx。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 x⩾0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)=ex−e−x−4x+2sinx,f′′(x)=ex+e−x−4+2cosx,f′′′(x)=ex−e−x−2sinx,f(4)(x)=ex+e−x−2cosx,
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)>2ex⋅e−x−2cosx=2(1−cosx),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0与x之间),即当 x ⩾ 0 x\geqslant0 x⩾0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,-\infty
解 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)∣]+2h∣f′′(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)在 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)
解 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<\xi
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)(b−a)+21F′′(ξ)(b−a)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)
解 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)=(x−a)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(x−a)f′(2a+x)−f(x)=21(x−a)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}<\xi
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)dt−21(x−a)[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(x−a)f′(x)=21[f(x)−f(a)]−21(x−a)f′(x)=21(x−a)[f′(η)−f′(x)],
其中 a < η < x a<\eta
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