函数<img src="https://static.zxtiku.com/wangke/imgs/topic/160acff7d74335c33b075f8a01e8de81.png"/> (<img src="https://static.zxtiku.com/wangke/imgs/topic/e9f631d39d73096a30754a10b6eae3df.png"/> 为常数)在<img src="https://static.zxtiku.com/wangke/imgs/topic/e0bc330c748f3aeb3261b93cb1aa8b33.png"/> 时的弹性为( ).