设x1、x2是区间[2，6]上的任意两个实数，且x1＜x2，则f（x1）-f（x2）=2x1−1−2x2−1=2[(x2−1)−(x1−1)] (x1−1)(x2−1) =2(x2−x1)(x1−1)(x2−1)．由2＜x1＜x2＜6，得x2-x1＞0，（x1-1）（x2-1）＞0...