四棱柱ABCD﹣A1B1C1D1的底面是平行四边形，M是AC与BD的交点．若 AB→ = a→ ， AD→ = b→ ， AA1→

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四棱柱ABCD﹣A₁B₁C₁D₁的底面是平行四边形，M是AC与BD的交点．若 AB→ = a→ ， AD→ = b→ ， AA1→ = c→ ，则 C1M→ 可以表示为（）

A： $\vec{a} + \vec{b} + \frac{1}{2} \vec{c}$

B： $- \frac{1}{2} \vec{a} - \frac{1}{2} \vec{b} + \vec{c}$

C： $- \frac{1}{2} \vec{a} - \frac{1}{2} \vec{b} - \vec{c}$

D： $\frac{1}{2} \vec{a} + \frac{1}{2} \vec{b} + \vec{c}$

上一题下一题 0.0难度选择题更新时间:2019-01-02 08:01:48

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Tom is a schoolboy.He is only seven years old, but he is very busy on weekdays.One Saturday he decided to relax himself, so he went to the cinema.

It was the first time for him to do that.He bought a ticket and then went in.But after two or three minutes he came out, bought a second ticket and went in again.After a few minutes he came out again and bought a third ticket.Two or three minutes later he came out and asked for another ticket.Then the girl in the ticket office asked him, “Why did you buy so many tickets? How many friends did you meet?” Tom answered, “No, I have no friends here.But a big boy always stops me at the door and tears(撕) my ticket.”

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表为人体吸入气体和呼出气体成分含量（%）比较；为了探究人体呼出气体中二氧化碳体积分数的变化，某同学做了实验，结果如图乙所示；人体内的气体可通过血液循环运输到组织细胞，血液和组织细胞间的气体交换如图乙示，请分析回答：

成分	吸入气体	呼出气体
氧气	20.96	16.40
二氧化碳	0.04	4.10
其他气体	79.00	79.5
合计	100.00	100.00

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