猜一猜

一棵小树五个杈，

不长叶子不开花。

灵灵巧巧本领大，

创造世界全靠它。

谜底是____。

阅读短文，下列问题

Computers are very important for the students now. For the city students, it's very easy to find a place to surf the Internet(网上冲浪). And for the village students, they can go to the Internet bar(网吧). Most of them spend more than two hours there, and some of them even spend more than eight hours there every day. They can do their homework, download(下载) some information for their homework, download music, send emails and play games.Most of them spend too much time playing games, so it's bad for their study.

（Ⅰ）若f（x）在 $(-\infty ,\frac{1}{2}$ 是减函数，在 $\frac{1}{2},+\infty )$ 是增函数，求实数a的值；

（Ⅱ）求实数a的取值范围，使f（x）在区间﹣5，5上是单调函数，并指出相应的单调性．