用方框中所给单词的适当形式填空，使短文正确，通顺。（每词限用一次）

                                                                      The population in China
   China has the____ population in the world, and about one ____ of the people in the world live in China. Because of the large____  , there is less____ space for each family. And it's difficult for lots of people____  jobs. The large population also causes many____ difficulties for the whole nation .For example, we're ____ of energy and water. Most cities are ____   than before, ____ the traffic is much heavier .The large population____ a serious problem.

关心社会，我们应该（  ）
①关心社会的发展变化
②明确公民责任与义务
③积极参与社会生活
④家事国事天下事，事事关心 

观察下列各式：

3×5=15=4²﹣1

5×7=35=6²﹣1

…

11×13=143=12²﹣1

…

（1）写出一个符合以上规律的式子．

（2）用字母表示一般规律，并说明该等式一定成立．