设圆的半径是r，则其摆线的一个拱的宽度与高度分别是____、____．

从下面方框中选择正确的单词并用其适当形式填空，使短文意思完整、通顺。

Dear Jenny,

    My name is Alan. I'm a 12-year- old girl. I'm the only ____ in my family. My birthday is on ____ 1st. It's the ____ day of the sixth month. It's also the Children's Day(儿童节). My parents ____ a party for me on that day. My friends ____ to the party. They bring(带来) me many birthday presents(礼物), and they sing Happy Birthday to me. ____ eat some strawberries, oranges and ____. We eat chicken and cakes ____ dinner. We also play games. We have a ____ time! How old are you? ____ is your birthday? Can you tell me?

                                                                                                                                                                                      Alan

计算a⁴•${\left(\frac{1}{a}\right)}^2$的结果是（　　）

——据吕思勉《魏晋南北朝史》