问题
选择题
已知
(1)|
(2)
(3)
(4)|
|
答案
∵
a |
b |
c |
若|
a |
b |
a |
b |
a |
b |
⇔|cosθ|=1
⇔cosθ=±1
⇔θ=0或θ=π
⇔
a |
b |
a |
b |
⇔θ=π
⇔cosθ=-1
⇔
a |
b |
a |
b |
a |
b |
⇔
a |
b |
⇔|
a |
b |
a |
b |
⇔|
a |
b |
a |
b |
若|
a |
b |
a |
c |
b |
c |
a |
c |
b |
c |
当|
a |
c |
b |
c |
a |
b |
c |
a |
b |
故(4)错误;
故选C
已知
(1)|
(2)
(3)
(4)|
|
∵
a |
b |
c |
若|
a |
b |
a |
b |
a |
b |
⇔|cosθ|=1
⇔cosθ=±1
⇔θ=0或θ=π
⇔
a |
b |
a |
b |
⇔θ=π
⇔cosθ=-1
⇔
a |
b |
a |
b |
a |
b |
⇔
a |
b |
⇔|
a |
b |
a |
b |
⇔|
a |
b |
a |
b |
若|
a |
b |
a |
c |
b |
c |
a |
c |
b |
c |
当|
a |
c |
b |
c |
a |
b |
c |
a |
b |
故(4)错误;
故选C