例:
I know this animal.
This year is the year of the dragon.
The dragon is the fifth.
如图,点A在双曲线上,且OA=4,过A作AC⊥轴,垂足为C,OA的垂直平分线交OC于B,则△ABC的周长为 ( )
已知a,b∈R,且a+b=5,则2a+2b的最小值是( )
人耳的听觉范围是____至____,频率高于____的声音叫做超声波,频率低于____的声音叫做次声.