|
时/式 |
一般 |
进行 |
完成 |
|
现在 |
am
is given
are |
am
is being
are |
has
been given
have |
|
过去 |
was
given
were |
was
being given were |
had been given |
|
将来 |
shall
be given
will |
|
shall
have been given
will |
|
过去将来 |
should
be given
would |
|
should
have been given
would |
21.(本题15分)
(Ⅰ)
,由OP⊥PQ,得
=0,
由
,得得cosa =
,
a < –2或a >2. 7分
(Ⅱ)(向量坐标法)当a= –1时,
,
|
(第21题) |
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当
,即
时,取等号.
又
在
上是减函数,
.
8分
另解:(余弦定理法).如图,
,
设
,则
,
又
在
上是减函数,
,
此时
,
8分