"本年度最费时及易错31题"中的数学问题

jupiter_li

"本年度最费时及易错31题"中的问题 1.2钥匙放入5个钥匙的钥匙环中,排在一起的概率 ANSWER 2C(5,1)/P(6,2) : 1/3 SURE 为什么是P6，2？？？ 10.the coin 抛三次，每次同一面概率：1/4 ?????应该是1/8啊：1/2*1/2*1/2 14.There are two sets, x and y . X is composed of multipples of 2, and y is comosed od multiples of 4, is it poosible for integer m be of x while not of y? (1) m is even (2) m/2 is not even integer ? 是选B吧？？？？？ 21.三角形，有图，画出三条高，问周长>1？ (1)两条高<1/3 (2)一条高>1/2 B why? 24.For all x, x is positive integer, \"2-height\" is defined to be the greatest nonnegtive n of x, what is the greatest number of 2-height when 2^n (2的n次方) is the factor of x? A. 2 B. 12 C. 40 D. 76 E. 90 C. 40（各选项质因数分解后，指数最大者） "2-height\" is defined to be the greatest nonnegtive n of x,这个定义对么？ 27.已知抛一种硬币落地的正反面概率不是1/2，问抛两次硬币都是正面朝上的 概率是多少？ 1) 每抛一次，正面朝上的可能性是反面的朝上的两倍 2) 抛两次，一正一反的概率是4/9 答案A 为什么B不行？？B是不是无解？

pgclt

1.2钥匙放入5个钥匙的钥匙环中,排在一起的概率
ANSWER 2C(5,1)/P(6,2)
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(5*2)/(5*6) = 1/3
10.the coin 抛三次，每次同一面概率：1/4
?????应该是1/8啊：1/2*1/2*1/2
-----------------------
同一面 can be either head or tail: 2* 1/8=1/4

14.There are two sets, x and y . X is composed of multipples of 2, and y is comosed od multiples of 4, is it poosible for integer m be of x while not of y?
(1) m is even
(2) m/2 is not even integer
? 是选B吧？？？？？
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(2) m/2 is not even integer => m is a multiple of 2 but not 4. Positive and sufficent.

tongxun

1、钥匙题：2钥匙放入5个钥匙的钥匙环中,排在一起的概率 有另一个方法：先放一个进去，这时有6个空，再放一个要与先放的相邻，只有2个空。 2/6=1/3 2、pgclt讲得详细。因为题目没说是那面。所以有2个面啊。 21.三角形，有图，画出三条高，问周长>1？ (1)两条高<1/3 (2)一条高>1/2 B why? 高>1/2 。由直角三角形斜边>直角边，可知三角形的两腰都>1/2 ，因此，不算底边周长已经>1了。 24、你的定义是对的。 27、条件2）推不出正反的具体比例。

cook

21.三角形，有图，画出三条高，问周长>1？ (1)两条高<1/3 (2)一条高>1/2 B why? ============================ Try to draw a real triangle, then I am sure you will find by using a=b>c 24.For all x, x is positive integer, \"2-height\" is defined to be the greatest nonnegtive n of x, what is the greatest number of 2-height when 2^n (2的n次方) is the factor of x? A. 2 B. 12 C. 40 D. 76 E. 90 C. 40（各选项质因数分解后，指数最大者） "2-height\" is defined to be the greatest nonnegtive n of x,这个定义对么？ ==================================================== Yes, it is right, given by ETS. 27.已知抛一种硬币落地的正反面概率不是1/2，问抛两次硬币都是正面朝上的 概率是多少？ 1) 每抛一次，正面朝上的可能性是反面的朝上的两倍 2) 抛两次，一正一反的概率是4/9 答案A 为什么B不行？？B是不是无解？ ====================================== Actually, under condition B, you will get two answer: 2/3 or 1/3 . That is why you can choose B.

cook

sorry, that 's why you can't choose B.

cook

14. Are you sure : (2) m/2 is not even integer => m is a multiple of 2 but not 4. Positive and sufficent ????
============================================
Why not m/2 is a fraction?

tongxun

maybe pgclt is too tired when he answered this question, I think.

wane

what`s the answer of No14?

pgclt

14.C. I was tired and in a rush. hope i won't be too tired when taking the real exam.

wane

那就有个问题，题目问的是is it possible,那1，2单独也是可能的把？