1,当a=0时,①b≧0时,f(x)单调递增;②b<0时,f(x)单调递减
2,当a>0时,x≧-b/2a,f(x)单调递增;x<-b/2a,f(x)单调递减
3,当a<0时,x≧-b/2a,f(x)单调递减;x<-b/2a,f(x)单调递增