f'(x)=2ax+1/x 切线斜率f'(e) 方程y=2aex-ae^2+x/e 过(0.5,1/2/e)
g(x)=f2(x)-f(x) g(1+)>0 a>-1/2 g'(x)=(1-2a+1/x)(x-1)
a