题目内容
(1)已知

(2)试确定一个区间D,


说明:对于第(2)题,将根据写出区间D所体现的思维层次和对问题探究的完整性,给予不同的评分.
【答案】分析:(1)利用正切化为正弦、余弦,和角公式求出cos(α+β)>0,根据
,推出α+β与
的大小.
(2)直接在
内找出一个子区间,区间是固定的,也可以是变化的,对任意的α、β∈D,当
时,恒有sinα<cosβ,利用函数的单调性,三角函数的符合特征,加以证明即可.
解答:解:(1)∵
∴
(2分)=>cos(α+β)>0(2分)
∵α+β∈(0,π)
∴
(2分)
(2)第一类解答:(1)若取
或取
等固定区间且D是
的子集并说明理由者给(2分),
(2)若取D=[γ1,γ2],
,并说明理由者给(3分)
理由:
若取
,
,
则-1<sinα<0,0<cosβ<1,即sinα<cosβ;
第二类解答:(1)若取
或取
等固定区间且D是
的子集,且解答完整得(4分)
(2)若取D是
的子集且区间的一端是变动者.且解答完整得(5分)
(3)若取D=[γ1,γ2],
,且解答完整得(6分)
取D=[γ1,γ2],
证明如下,设α,β∈[γ1,γ2],
,
又
,
则
,
因为-γ2≤-β≤γ1,
,
而
,
,
即:
,于是由α,β∈[γ1,γ2],
,且
以及正弦函数的单调性得:
,即:0<sinα<cosβ
第三类解答:
(1)若取
或取
等固定区间且D是
的子集(两端需异号),且解答完整得(6分)
(2)若取D是
的子集且区间的一端是变动者(两端需异号).且解答完整得(7分)
(3)若取取D=[γ1,γ2],
,(γ1与γ2需异号)且解答完整得(8分)
若取
,
因为:
,
,
则
亦有:
,
这时,
,
,
而
为
,
所以有sinα<cosβ.
(如出现其它合理情况,可斟酌情形给分,但最高不超过8分).
点评:本题考查比较大小,正弦函数的单调性,考查分析问题解决问题的能力,是中档题.


(2)直接在


解答:解:(1)∵

∴

∵α+β∈(0,π)
∴

(2)第一类解答:(1)若取



(2)若取D=[γ1,γ2],

理由:
若取


则-1<sinα<0,0<cosβ<1,即sinα<cosβ;
第二类解答:(1)若取



(2)若取D是

(3)若取D=[γ1,γ2],

取D=[γ1,γ2],

证明如下,设α,β∈[γ1,γ2],

又

则

因为-γ2≤-β≤γ1,

而


即:



以及正弦函数的单调性得:

第三类解答:
(1)若取



(2)若取D是

(3)若取取D=[γ1,γ2],

若取

因为:


则

亦有:

这时,


而


所以有sinα<cosβ.
(如出现其它合理情况,可斟酌情形给分,但最高不超过8分).
点评:本题考查比较大小,正弦函数的单调性,考查分析问题解决问题的能力,是中档题.

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