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