设向量 a→ =（1，cosθ））与 b→ =（﹣1，2cosθ）垂直，则cos2θ等于

题干

设向量 a→ =（1，cosθ））与 b→ =（﹣1，2cosθ）垂直，则cos2θ等于（）

A：0

B： $\frac{1}{2}$

C： $\frac{\sqrt{2}}{2}$

D：﹣1

上一题下一题 0.0难度选择题更新时间:2017-08-29 10:02:36

A

Hello, my name is Betty and I'm in Class Six. I'm twelve years old. I'm American. I'm from New York. Mike is in my class and he is my good friend.

Hi, I'm Mike. I'm from London. I'm in Class Six, too. Wang Tao is my friend. He is in my class.

Hi, my name is Wang Tao. I'm Chinese. I'm from Hangzhou. I'm in Class Six with Betty and Mike. They are my good friends.