|a+b|+|a-2b|大于等于|a|(|x-1|+|x-2|)|1+b/a|+|1-2b/a|大于等于(|x-1|+|x-2|)|1+b/a|+|1-2b/a|最小值是x=1/2 ymin=3/2(|x-1|+|x-2|)<=3/23/4<=x<=9/4
