题目

 There seems never to have been a civilization(文明) without toys, but when and how they developed is unknown. They probably came about just to give children something to do. In the ancient world, as is today, most boys play with some kinds of toys and most girls with another. In societies where social roles are strictly determined, boys pattern their play after the activities of their fathers and girls after the tasks of their mothers. This is true because boys and girls are being prepared, even in play, to step into the roles and responsibilities of the adult world. What is re“约去”指数：如33+1333+23=3+13+2，53+2353+33=5+25+3，…你见过这样的约分吗？面对这荒谬的约分，一笑之后，再认真检验，发现其结果竟然正确！这是什么原因？仔细观察式子，我们可作如下猜想：a3+b3a3+(a-b)3=a+ba+(a-b)，试说明此猜想的正确性．（供参考：x3+y3=（x+y）（x2-xy+y2））