易证△ABC≌△BDE，从而得到△BCD的面积为 $\frac{1}{2}{a}^2$ ．

初步探究：如图②，在Rt△ABC中，∠ACB=90°，BC=a．将边AB绕点B顺时针旋转90°得到线段BD，连结CD．用含a的代数式表示△BCD的面积，并说明理由．

简单应用：如图③，在等腰三角形ABC中，AB=AC，BC=a．将边AB绕点B顺时针旋转90°得到线段BD，连结CD．直接写出△BCD的面积．（用含a的代数式表示）

一个人晨练，按如图所示，走半径为R的中国古代的八卦图的路线，中央的S形部分是两个直径为R的半圆．BD、CA分别为西东、南北指向．他从A点出发沿曲线ABCOADC运动．求：

方程x=x（x-4）的解是（　　）

B

    My name is Alice. I have four aunts. They are all great women. Now, let's learn about them.

    This is Emily Smith. She is my father's sister. She is twenty-eight years old. She works at a big computer company.

    This is Rachel Smith. She is my father's sister, too. She is thirty years old. She is a doctor. She takes care of （照顾）patients in a hospital. This is her office. There is a little girl in front her.

    This is Julia King. She is my mother's sister. She is forty-five years old. She is a scientist（科学家）. She works in a lab. It's not big but it's really nice.

    This is Alison King. She is my mother's sister too. She is thirty-two years old. She is a policewoman. Her job is at a police station.

将自由下落的物体下落的高度自上而下分成三段，则以下看法错误的是（   ）