题目

We are surrounded by mixed messages about mistakes: we're told we earn by making them, but we work hard to avoid them. So the result is that most of us know that we are going to make mistakes, but deep down, we feel we shouldn't.     Experiments with schoolchildren who did well on a given test show that those who were praised for being smart and then offered a more challenging or less challenging task afterward usually chose the easier one. On the other hand, children praised for trying hard-rather than being smart-far more often selected the more difficult task.     If we try har设2a=3，2b=6，2c=12，则a，b，c成 数列．