甲数比乙数少15,乙数是30,甲数是____。
① a→ 与 b→ 的夹角;
②求| a→ + b→ |和| a→ ﹣ b→ |.
读一读,写一写
①— Are there any ____ ?
— Yes, there are.
②There are many tall ____ in my city.
③There is a ____ above the river.
④- Are there any houses in the ____?
-Yes, there are.
尺规作图:画出线段AB的垂直平分线(不写作法,保留作图痕迹)