f(x)=X平方+alnx
零和负数无对数,定义域x＞0
a=-2时,f(x) = x^2 - 2lnx
f'(x) = 2x -2/x = 2(x^2-1)/x = 2(x+1)(x-1)/x
x∈（0,1）时f'(x)＜0,单调减；
x∈（1,+∞）时f'(x)＞0,单调增
极小值f(1)=1^2-0 = 1