f(x)的定义域是x>0
f`(x)=2ax+(a+2)+1/x
=(2ax^2+(a+2)x+1)/x
=(ax+1)(2x+1)/x
当a>=0时
f`(x)>0
f(x)在（0,+∞）上单调增
a<0时
令f`(x)>=0
x<=-1/2或x>=-1/a
∵x>0
∴f(x)在（0,-1/a）上单调减
f(x)在[1/a,+∞）上单调增