句型转换。

1. John answered all the questions. (改为被动语态)

    All the questions _____          _ _____ John.

2. They are too tired to continue working. (改为同义句)

    They are _____ _____ _____  they can't continue working.

3. His brother joined the army when he was 22. (改为同义句)

    His brother joined the army _____ _____ _____ _____ 22.

4. What a tall tree this is! (用how改写)

    _____ _____ _____ tree this is!

5. Some people believe that Chinese is difficult to learn. (改为同义句)

    It _____ _____ _____ Chinese is difficult to learn.

人们利用发电机把天然存在的各种形式的能量转化为电能，为了合理地利用这些能源，发电站要修建在靠近这些天然资源的地方．但用电的地方却分布很广，因此需要把电能输送到很远的地方．某电站输送电压U=5000V，输送功率P=500kW，安装在输电线路的起点和终点的电度表一昼夜读数相差4800kW•h．试求：

（1）该电路的输电效率和输电线的电阻．

（2）如果发电机输出电压为250V，用户需要电压为220V．要使输电损耗为5%，分别求升压和降压变压器的原、副线圈匝数之比．（计算时取