如图所示，已知∠1=∠2，∠3=∠4，∠5=∠C，求证：DE//BF

$\alpha ={60}^0$是“$\cos \alpha =\frac{1}{2}$”的(         )

如图所示，一小球从一半圆轨道左端A点正上方某处开始做平抛运动（小球可视为质点），飞行过程中恰好与半圆轨道相切于B点．O为半圆轨道圆心，半圆轨道半径为R，OB与水平方向夹角为60°，重力加速度为g，则小球抛出时的初速度为（   ）

根据短文阅读理解，选择正确答案。

      A good way to pass an examination(考试) is to work hard every day in the year. You may fail in the examination if you are lazy for most of the year and then work hard only a few days before the examination. If you are taking an English examination, don’t learn only rules of grammar.

      Try to read stories in English and speak in English whenever you can. A few days before examination you should start going to bed early. Do not stay up late at night studying and learning things. Before you start the examination, read carefully over the examination paper. Try to understand the exact meaning of each question before you pick up your pen to write. When you have at last finished your examination, read your answers, correct any mistake that you see and make sure that you have not missed anything out.