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专为“同构”这碟醋包的饺子(2022新高考Ⅰ导数)

2022-10-06 23:32 作者:数学老顽童  | 我要投稿


解:(1)若a%5Cleqslant%200,则f%5Cleft(%20x%20%5Cright)%20g%5Cleft(%20x%20%5Cright)%20%5Csearrow%20

无最小值,不合题意.

a%3E0时,

f'%5Cleft(%20x%20%5Cright)%20%3D%5Cmathrm%7Be%7D%5Ex-a

f'%5Cleft(%20x%20%5Cright)%20%3D0,得x%3D%5Cln%20%20a

其单调性如下表所示:

%5Cbegin%7Barray%7D%7Bc%7Cc%7Cc%7D%0A%09x%26%09%09%5Ccolor%7Bred%7D%7B%5Cleft(%20-%5Cinfty%20%2C%5Cln%20a%20%5Cright)%7D%26%09%09%5Ccolor%7Bred%7D%7B%5Cleft(%200%2C%2B%5Cinfty%20%5Cright)%7D%5C%5C%0A%09%5Chline%0A%09f%5Cprime%5Cleft(%20x%20%5Cright)%26%09%09-%26%09%09%2B%5C%5C%0A%09%5Chline%0A%09f%5Cleft(%20x%20%5Cright)%26%09%09%5Ccolor%7Bred%7D%7B%5Csearrow%7D%26%09%09%5Ccolor%7Bred%7D%7B%5Cnearrow%7D%5C%5C%0A%5Cend%7Barray%7D

%5Ccolor%7Bred%7D%7Bf%5Cleft(%20x%20%5Cright)%20_%7B%5Cmin%7D%7D%3Df%5Cleft(%20%5Cln%20%20a%20%5Cright)%20%3D%5Ccolor%7Bred%7D%7Ba-a%5Cln%20%20a%7D.

g'%5Cleft(%20x%20%5Cright)%20%3Da-%5Cfrac%7B1%7D%7Bx%7D

g'%5Cleft(%20x%20%5Cright)%20%3D0,得x%3D%5Cfrac%7B1%7D%7Ba%7D

其单调性如下表所示:

%5Cbegin%7Barray%7D%7Bc%7Cc%7Cc%7D%0A%09x%26%09%09%5Ccolor%7Bred%7D%7B%5Cleft(%200%2C%5Cfrac%7B1%7D%7Ba%7D%20%5Cright)%7D%26%09%09%5Ccolor%7Bred%7D%7B%5Cleft(%20%5Cfrac%7B1%7D%7Ba%7D%2C%2B%5Cinfty%20%5Cright)%7D%5C%5C%0A%09%5Chline%0A%09g%5Cprime%5Cleft(%20x%20%5Cright)%26%09%09-%26%09%09%2B%5C%5C%0A%09%5Chline%0A%09g%5Cleft(%20x%20%5Cright)%26%09%09%5Ccolor%7Bred%7D%7B%5Csearrow%7D%26%09%09%5Ccolor%7Bred%7D%7B%5Cnearrow%7D%5C%5C%0A%5Cend%7Barray%7D

%5Ccolor%7Bred%7D%7Bg%5Cleft(%20x%20%5Cright)%20_%7B%5Cmin%7D%7D%3Dg%5Cleft(%20%5Cfrac%7B1%7D%7Ba%7D%20%5Cright)%20%3D%5Ccolor%7Bred%7D%7B1%2B%5Cln%20%20a%7D.

依题意:a-a%5Cln%20%20a%3D1%2B%5Cln%20%20a

%5Cln%20%20a%2B%5Cfrac%7B2%7D%7Ba%2B1%7D-1%3D0.

h%5Cleft(%20a%20%5Cright)%20%3D%5Cln%20%20a%2B%5Cfrac%7B2%7D%7Ba%2B1%7D-1,则

%5Ccolor%7Bred%7D%7Bh'%5Cleft(%20a%20%5Cright)%7D%20%3D%5Cfrac%7B1%7D%7Ba%7D-%5Cfrac%7B2%7D%7B%5Cleft(%20a%2B1%20%5Cright)%20%5E2%7D%3D%5Cfrac%7Ba%5E2%2B1%7D%7Ba%5Cleft(%20a%2B1%20%5Cright)%20%5E2%7D%5Ccolor%7Bred%7D%7B%3E0%7D

h%5Cleft(%20a%20%5Cright)%20%5Ccolor%7Bred%7D%7B%5Cnearrow%20%7D

h%5Cleft(%20a%20%5Cright)%20最多仅有一个零点

又因为h%5Cleft(%201%20%5Cright)%20%3D0,故%5Ccolor%7Bred%7D%7Ba%3D1%7D.

(2)由(1)可知

f%5Cleft(%20x%20%5Cright)%20%3D%5Cmathrm%7Be%7D%5Ex-xg%5Cleft(%20x%20%5Cright)%20%3Dx-%5Cln%20%20x

%5Cmathrm%7Be%7D%5Ex-x%5Cgeqslant%201x-%5Cln%20%20x%5Cgeqslant%201

所以%7B%5Ccolor%7Bred%7D%20%7B%5Cln%20%20x%3Cx%3C%5Cmathrm%7Be%7D%5Ex%7D%7D

并且%7B%5Ccolor%7Bred%7D%20%7Bf%5Cleft(%20%5Cln%20%20x%20%5Cright)%20%3Dg%5Cleft(%20x%20%5Cright)%20%7D%7D%7B%5Ccolor%7Bred%7D%20%7Bg%5Cleft(%20%5Cmathrm%7Be%7D%5Ex%20%5Cright)%20%3Df%5Cleft(%20x%20%5Cright)%20%7D%7D

h%5Cleft(%20x%20%5Cright)%20%3Df%5Cleft(%20x%20%5Cright)%20-g%5Cleft(%20x%20%5Cright)%20x%5Cin%20%5Cleft(%200%2C%2B%5Cinfty%20%5Cright)%20

x%5Cin%20%5Cleft(%201%2C%2B%5Cinfty%20%5Cright)%20时,%5Cln%20%20x%5Cin%20%5Cleft(%200%2C%2B%5Cinfty%20%5Cright)%20

x%5Cln%20%20x同属于区间%5Cleft(%200%2C%2B%5Cinfty%20%5Cright)%20

因为f%5Cleft(%20x%20%5Cright)%20在该区间%5Cnearrow%20,所以:

h%5Cleft(%20x%20%5Cright)%20%3Df%5Cleft(%20x%20%5Cright)%20-%7B%5Ccolor%7Bred%7D%20%7Bg%5Cleft(%20x%20%5Cright)%20%7D%7D%3Df%5Cleft(%20x%20%5Cright)%20-%7B%5Ccolor%7Bred%7D%7B%20f%5Cleft(%20%5Cln%20%20x%20%5Cright)%7D%20%7D%3E0

x%5Cin%20%5Cleft(%200%2C1%20%5Cright)%20,易知h%5Cleft(%20x%20%5Cright)%20%5Cnearrow%20

又因为

%5Cbegin%7Baligned%7D%0A%09h%5Cleft(%20%5Cfrac%7B1%7D%7B%5Cmathrm%7Be%7D%7D%20%5Cright)%20%26%3D%5Cmathrm%7Be%7D%5E%7B%5Cfrac%7B1%7D%7B%5Cmathrm%7Be%7D%7D%7D-%5Cleft(%201%2B%5Cfrac%7B2%7D%7B%5Cmathrm%7Be%7D%7D%20%5Cright)%5C%5C%0A%09%26%3C%5Cmathrm%7Be%7D%5E%7B%5Cfrac%7B1%7D%7B%5Ccolor%7Bred%7D%7B2%7D%7D%7D-%5Cleft(%201%2B%5Cfrac%7B2%7D%7B%5Ccolor%7Bred%7D%7B3%7D%7D%20%5Cright)%5C%5C%0A%09%26%3D%5Csqrt%7B%5Cmathrm%7Be%7D%7D-%5Cfrac%7B5%7D%7B3%7D%3C0%5C%5C%0A%5Cend%7Baligned%7D

h%5Cleft(%201%20%5Cright)%20%3D%5Cmathrm%7Be%7D-2%3E0

存在唯一x_0%5Cin%20%5Cleft(0%2C%2B%5Cpropto%20%20%5Cright)%20,使h%5Cleft(%20x_0%20%5Cright)%20%3D0

%5Ccolor%7Bred%7D%7Bf%5Cleft(%20x_0%20%5Cright)%20%3Dg%5Cleft(%20x_0%20%5Cright)%20%7D

%5Cbbox%5B%23def%2C5px%2Cborder%3A0px%20solid%5D%7B%0A%5Cmathrm%7Be%7D%5E%7Bx_0%7D%2B%5Cln%20%20x_0%3D2x_0%7D.

又因为:%7B%5Ccolor%7Bred%7D%20%7Bf%5Cleft(%20%5Cln%20%20x_0%20%5Cright)%20%3Dg%5Cleft(%20x_0%20%5Cright)%20%7D%7D

%7B%5Ccolor%7Bred%7D%20%7Bg%5Cleft(%20%5Cmathrm%7Be%7D%5E%7Bx_0%7D%20%5Cright)%20%3Df%5Cleft(%20x_0%20%5Cright)%20%7D%7D,

所以:%5Ccolor%7Bred%7D%20%7Bf%5Cleft(%20%5Cln%20%20x_0%20%5Cright)%20%3Df%5Cleft(%20x_0%20%5Cright)%20%7D

%5Ccolor%7Bred%7D%20%7Bg%5Cleft(%20%5Cmathrm%7Be%7D%5E%7Bx_0%7D%20%5Cright)%20%3Dg%5Cleft(%20x_0%20%5Cright)%20%7D.

因为%5Cln%20%20x_0%5Cne%20x_0%5Cne%20%5Cmathrm%7Be%7D%5E%7Bx_0%7D

故存在直线y%3Db,其与两条曲线y%3Df%5Cleft(%20x%20%5Cright)%20y%3Dg%5Cleft(%20x%20%5Cright)%20共有三个不同的交点:%5Cleft(%20%7B%5Ccolor%7Bred%7D%20%7B%5Cln%20%20x_0%7D%7D%2Cb%20%5Cright)%20%5Cleft(%20%7B%5Ccolor%7Bred%7D%7B%20x_0%7D%7D%2Cb%20%5Cright)%20%5Cleft(%20%7B%5Ccolor%7Bred%7D%20%7B%5Cmathrm%7Be%7D%5E%7Bx_0%7D%7D%7D%2Cb%20%5Cright),且%5Cbbox%5B%23def%2C5px%2Cborder%3A0px%20solid%5D%7B%0A%5Cmathrm%7Be%7D%5E%7Bx_0%7D%2B%5Cln%20%20x_0%3D2x_0%7D.

即三个交点的横坐标成等差数列.


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