3道prep1上答案有纠结的题

tiya_1125

1. in the figure above, equilateral triangle ABC is inscribed in the circle, if rhe length of arc ABC is 24. what is the approximate diameter of the circle?
A 5   B 8   C 11 D 15  E  19  

这道题我的解法是：因为是内接等边三角形， 则弧长x 3 则是圆的周长。 即 24*3 = 2*pai*r 。 因为文中问的是diameter 直径，所以24*3/pai就可以了，答案20左右，所以我选19。但本题的正确答案是c11， 即是半径了。很困惑。


2. A basket contains 5 apples, of which 1 is spoiled and the rest are good. If Henry is to select 2 apples from the basket simultaneously, and at random, which is the probability that the 2 apples selected will include the spoiled apple?

A 1/5   B 3/10   C 2/5    D 1/2    E 3/5  
我选A， 但答案选择D



3. Alice 's take home pay last year was the sam eeach month, and she saved the same fraction of her take-home pay each month. The total amount of money she had saved at the end of the year was 3 times the amount of that portion of her monthly take -home pay that she did not save. If all the money she saved last year was from her take-home pay, what fraction of her take-home pay did she saved each month?
A 1/2   B 1/3    C 1/4    D 1/5   E 1/6
我选c， 答案选d


大家帮着解答一下哈！！！

alanuonuo

哎呀楼主，第一题明显弧长ABC是三分之二个圆嘛

alanuonuo

第二题答案是C吧？怎么会是D呢？
我的想法：拿到两个都是好苹果的概率是4/5×3/4=3/5，那么其中有一个是坏苹果的概率就是1-3/5=2/5；或者共有C上2下5=10种取法，拿到有坏苹果的取法是C上1下4×C上1下1=4种，概率为4/10=2/5

对否？

alanuonuo

3. 假设她每月存钱为X，每月税后工资为1，根据题意可得（1-X）×3=12X，得出X=1/5

tiya_1125

有图的。就是一个圆里内接一个等边3角形。为什末是2/3弧？我觉得是1/3弧啊。

tiya_1125

第一题你说的对！我做糊涂了。

tiya_1125

挑苹果的题我又算了一下，用的是减法原理。
答案是c， 没有错。多谢了！

winow

同问第一题

第二题同意C……
不是吧prep答案也有错？

第三题
saved 的比上没saved 的是1/4
与take-home pay 比就是1/5啦

alanuonuo

我知道啊，我做过第一题了，你要想一下，如果是1/3弧的话，应该是直接说弧AC了，弧ABC就是从A点经过B点再到C啊。

winow

我知道啊，我做过第一题了，你要想一下，如果是1/3弧的话，应该是直接说弧AC了，弧ABC就是从A点经过B点再到C啊。

-- by 会员 alanuonuo (2010/8/4 10:17:01)

理解了~~ thank you