首都师范大学2020数学分析
证明:
由
$\displaystyle (x+y)^\alpha -x^\alpha -y^\alpha =x^\alpha [(1+\frac{y}{x})^\alpha -1-(\frac{y}{x})^\alpha],x^\alpha >0,$
可令
$\displaystyle f(t)=(1+t)^\alpha -1-t^\alpha,t=\frac{y}{x}> 0,$
显然有
$\displaystyle f(0)=0.$
而
$\displaystyle f'(t)=\alpha (1+t)^{\alpha-1}-\alpha t^{\alpha-1}=\alpha [(1+t)^{\alpha-1}-t^{\alpha-1}],$
再令
$\displaystyle g(t)=t^{\alpha -1},$
则有
$\displaystyle g'(t)=(\alpha -1)t^{\alpha -2}< 0,$
$\displaystyle \therefore \Rightarrow f'(t)< 0,\Rightarrow f(t)\downarrow ,$
$\displaystyle \therefore f(t)=(1+t)^\alpha -1-t^\alpha< f(0)=0.$
从而有
$\displaystyle (x+y)^\alpha -x^\alpha -y^\alpha =x^\alpha [(1+\frac{y}{x})^\alpha -1-(\frac{y}{x})^\alpha]< 0,$
即
$\displaystyle (x+y)^\alpha < x^\alpha +y^\alpha.$