题目

Group–buying in China Modeled after US hot website Groupon.com, group buying websites are now popular in China. These websites use the power of group buying to get competitive discounts (打折)for a daily deal on some best stuff to do, see, eat and buy in the cities across China. Discounts are available within just one click. Zhao lei, a software engineer in Beijing, loves the one-hour lunch break at noon, as it is the best time for him to check “today’s special” at his favorite group buying websites . Sometimes he searches for great deals at directory sites devoted to the new 在R+上的递减函数f（x）同时满足：（1）当且仅当x∈M?R+时，函数值f（x）的集合为[0，2]；（2）f（）=1；（3）对M中的任意x1、x2都有f（x1•x2）=f（x1）+f（x2）；（4）y=f（x）在M上的反函数为y=f-1（x）．（1）求证：∈M，但∉M；（2）求证：f-1（x1）•f-1（x2）=f-1（x1+x2）；（3）解不等式：f-1（x2-x）•f-1（x-1）≤．