f(x)=x lnx
f(x) is defined for x>0
f(x)=x lnx
f'(x) = 1+ lnx =0
x=1/e
f''(x) =1/x >0
f(x)
减小 =(0,1/e]
增加=[1/e,无穷 ）
minf(x) = f(1/e) = -1/e