ＧＷＤ28中的题，求高人

selinamba

数学超烂滴偶，又来求助高人哩。。。120&47素昨天问滴，已经有高人解答哩。。偶粉感谢之。。决定不擦。。。留给和偶一级的童鞋看看吧，很好的解答都素。。。再来几道ＧＷＤ滴，感谢ing。。。

其实都有答案了，就这道，谢谢高人！！！！
ＧＷＤ28－10
Six cards numbered from 1 to 6 are placed in an empty bowl.  First one card is drawn and then put back into the bowl; then a second card is drawn.  If the cards are drawn at random and if the sum of the numbers on the cards is 8, what is the probability that one of the two cards drawn is numbered 5 ?
答案：2 /5，我怎么也算不出，哎，概率呀。。。。泪。。。


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ＧＷＤ28－18
The function f is defined by f(x) = √x – 10 for all positive numbers x.  If u = f(t) for some positive numbers t and u.  What is t in terms of u?


A.    √(u+10)
B.    (√u + 10)²
C.    √(u²+10)
D.    (u + 10)²
E.    (u² + 10)²

这道题要偶干嘛？？？看不懂，怎么代？？？


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ＧＷＤ28－28
Each of the 30 boxes in a certain shipment with either 10 pounds or 20 pounds, and the average (arithmetic mean) weight of the boxes in the shipment is 18 pounds.  If the average weight of the boxes in the shipment is to be reduced to 14 pounds by removing some of the 20-pound boxes, how many 20-pound boxes must be removed?


A.    4
B.    6
C.    10
D.    20
E.    24
答案选D,求解题思路


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ＧＷＤ28－29The sum of all the integers k such that –26 < k < 24 is


A.    0
B.    -2
C.    -25
D.    -49
E.    -51


答案选D，为虾米。。。题都看不懂。。。


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ＧＷＤ28－36
If a, b, c, and d are positive integers, is (a/b) (c/d) > c/b?
(1)    c > b
(2)    a > d
   
A. Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient.
B. Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient.
C. BOTH statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient.
D. EACH statement ALONE is sufficient.
E. Statements (1) and (2) TOGETHER are NOT sufficient.


答案选Ｂ，上面这个式子，怎么变，可以看出（2）是推的出这个结论滴哇？？？



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120  x-y=8, X>4?   (1)x+y<0   (2)xy<0  
这道题我是画图做的，感觉比一般的难一些，但是只要直接画图就简单多了。答案应该是A
同意。x+y<0可知x<4，所以充分。

狗狗的主人说画图，我愣没想出画什么图能做呀。。。

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再问一道，米办法，偶的排列组合实在太菜了、、、谢谢高人指点。。。感激不尽。。。
47  说有6个三人组的代表团开会，每人都握手，来自同一个代表团的不互相握，不同代表团的成员只握一次就好， 问一共多少次
我选135
做这种题思路的关键在哪呀？？？迷茫。。。

cenya

用y=x-8代入解题，不需要画图

selinamba

感谢之???偶都快想破脑袋哩???3QQQing~~

lasintin

那个排列组合滴可以用两种方法。

第一种：6个组，每组三人，一共有18个人，因为握手都是一对一，所以先从18个人任意选两个人C（2，18），这是全部人握手的数目，又因为每组间不能握手，则每组三个人任意选两个C（2，3），一共有六组为6*C（2，3），再用前面的减就可以了
最终公式：C（2，18）-6*C（2，3）=135

第二种：笨方法，直接算。第一组随便挑一个人跟后面5个组所有人握手有5*3，一个组三个人为3*5*3，第二组与第一组一样做法，有3*4*3，依次下来最后公式为：3*5*3+3*4*3+3*3*3+3*2*3+3*1*3=135

selinamba

太感谢哩，懂哩，懂哩。。。粉好，粉清楚，再次感谢ing~~ＣＤ高人就素多。。。强。。。彻底懂哩。。

selinamba

等复高人解答中。。。。

csdianna

ＧＷＤ28－18
The function f is defined by f(x) = √x – 10 for all positive numbers x.  If u = f(t) for some positive numbers t and u.  What is t in terms of u?


A.√(u+10)
B.(√u + 10)²
C.√(u²+10)
D.(u + 10)²
E.    (u² + 10)<sup>2



这题说白了就是要你用y来表示X，只是换了下字母，变成u和t了~</sup>

u=f(t) = √t – 10 ,   所以  u+10=√t  ,   所以 t=(u + 10)²


第一题不懂~我算出来是2/7，等高人指教~
~

csdianna

ＧＷＤ28－29The sum of all the integers k such that –26 < k < 24 is


A.    0
B.-2
C.-25
D.-49
E.-51


这题是要求从-25到23的等差数列的和，直接代公式：（-25+23）*49/2=-49
注意，-25和23之间有49个数，别把0丢了~

简便的方法就是-25-24-23-22……+22+23，前面的-23和+23抵消，-22和+22抵消，依次类推，就只剩-25-24=-49

shingshinglee

第10题~要和为8的情况~并且考虑顺序~
2~6
3~5
4~4
5~3
6~2
所以是2/5吧~
估计是这样的~不确定

shingshinglee

36题~
(a/b) (c/d) > c/b
=>(ac)/(bd)>c/b
=>a/d是不是大于1就可以了~
又都是正数~所以B