题目

    Once upon a time, there used to be a boy who was very rich and clever but nothing could make him feel happy．He had almost everything a boy could ever want，so he was only 46  in the most rare objects. One day his parents bought him a very strange   47  ．When the boy went to see his reflection(映像)in it，he looked very  48  ．He tried smiling，  49   his reflection continued with its sad expression.       50  , the boy went off to buy sweets and lots of toys．He came home as happy as he could be，but he   51  looked sad in that mirror.     先化简，再求值：，其中a=b．