空降兵某部官兵使用新装备进行超低空跳伞，若跳伞空降兵在离地面224m高处，由静止开始在竖直方向做自由落体运动．一段时间后立即打开降落伞，以12.5m/s²的加速度匀减速下降．为了空降兵的安全，要求空降兵落地速度最大不得超过5m/s．则空降兵打开降落伞时离地的高度至少为多少米？（g取10m/s²，不计空气阻力）

把两块含45°角的直角三角板按图1所示的方式放置，点D在BC上，连结BE、AD，AD的延长线交BE于点F．

阅读理解

(D)

    It's good to share your feelings with others. No matter whether your feelings are good or not, it can also help you to get closer to others.

    But how to share your feelings? First, you should pay attention to your feelings. What troubles you? Does it make you sad or angry? Do you have this sense only once or many times? You can know about your feelings in your head by writing them down on a piece of paper or even by drawing pictures. When you do that, it might help you to remember something about how it makes you feel. Then you can say "I feel sad because my friend won't play with me." or "I feel angry with my classmates..."     Do you know why you should talk about your feelings? If you always keep your feelings especially sadness, nervousness or anxiety (焦虑),they can even make you feel sick. But if you talk with someone, like your mom and dad, you'll feel much better. Your parents will be glad to know what's hapening in your life because they love you. But if you don't want to talk with them, please talk with your friends. They will help you.

已知 的解为 ，则 等于（　　）

I'm very proud that Beijing is one of _______ cities in the world.