（情境）在十二届全国人大二次会议上，有代表就当下我国公民读书学习现状提出担忧，“读书少”、“去经典化”、“重娱乐”、出现提笔忘字现象等，呼吁国家尽快制定相关制度扭转不良现象的发展，努力实现文化强国的目标。
阅读材料回答：

Dear Abby，

    I'm a helpless mother. My son Tom is 15 years old. I find that the older he grows, the less we talk. I feel very sad and I really need help.

    My first problem is about his hobbies. He spends almost all his spare time on computer games. When he gets home, he always turns on the computer and closes the door. I'm getting worriedabout him, especially when his English teacher tells me he always gets low marks in the English exams.

    Second, he likes newtechnology，but I don't have enough money and I can't afford all the things he wants.

    Recently, I've foundthat he is always alone. I've never seen him talking to his friends on the phone or going out with anyone on weekends. That is the last problem I have.

    What should I do to help him? Pleasegive me some advice.

A helpless mother

（Ⅰ）求数列{a_n}的通项公式；

（Ⅱ）设数列{ $\frac{1}{a_n{a}_{n+1}}$ }的前n项和为S_n，求证： $\frac{1}{4}$ ≤S_n＜ $\frac{1}{3}$ ．