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给出定义:若函数f(x)在D上可导,即f′(x)存在,且导函数f′(x)在D上...

 给出定义:若函数f(x)在D上可导,即f′(x)存在,且导函数f′(x)在D上也可导,则称f(x)在D上存在二阶导函数,记f″(x)=(f′(x))′.若f″(x)<0在D上恒成立,则称f(x)在D上为凸函数.以下四个函数在(0,)上是凸函数的是________.

(把你认为正确的序号都填上)

①f(x)=sinx+cosx;②f(x)=lnx-2x;③f(x)=-x3+2x-1;④f(x)=xex.

 

 

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