皮筋绷在纸盒上,绷得紧,振动发出的声音____;绷得松,振动发出的声音____。
The two boys lost their bags.That's they look sad.
将等腰Rt△ABC绕点A逆时针旋转15°得到△AB′C′,若AC=1,则图中阴影部分面积为( )
4+____=6