请教大家四道prep的题! 麻烦大家了

mimioy · 发表于 2010-8-3 00:27:42

1. What is the greatest possible area of a triangular region with one vertex at the center of a circle of a radius 1 and the other two vertices on the circle? key：1/2

2. Mary bought several pencils. If each was either a 23-cent pencil or 21-cent pencil, how many 23-cent pencils did Mary buy?
(1) Mary bought a total of 6 pencils.
(2) The total value of the pencils Mary bought was 130 cent. key: B

3. A certain jar contains only b black marbles, w white marbles, and r red marbles. If one marble is chosen at random from the jar, is the probability that the marble chosen will be red greater than the probability that the marble chosen will be white?
(1) r/(b+W) > w/(b+r)
(2) b-w>r key A

4. Jane bought some paperback books that cost $8 each and some handcover books that cost $25 each. If Jane bought more than 10 paperback books, how many hardcover books did he buy?
(1) The total cost of the handcover that bought was at least $150
(2) The total cost of the books that bought was less than $260.
我想问下at least 到底是> 还是>= ?? 我是当做>=, 结果1+2答案不唯一。。。key:C

mimioy · 发表于 2010-8-3 00:31:46

恳请各位指点迷津！

mimioy · 发表于 2010-8-3 00:38:09

为什么没有人讲一下。。。：（顶~~

叶子mj · 发表于 2010-8-3 00:42:41

第二题是23x+21y=130 且x y是正整数尾数是1和3能凑成10的结果不多可以自己试试我算的是x=2 y=4 应该没有别的可能了吧所以充分

mimioy · 发表于 2010-8-3 00:49:07

谢谢：）可以再看下另外3道吗。。。

alanuonuo · 发表于 2010-8-3 01:26:38

呵呵，现学现卖一下，第一题，S=1/2×R×R×sinA（夹角），sin的最大值为1，所以最大面积为1/2

alanuonuo · 发表于 2010-8-3 01:34:51

3. 红色的概率要大于白色，则r>w
条件一： r/(b+w) > w/(b+r)-->r(b+r)>w(b+w)-->-b(w-r)>(w+r)(w-r)-->(w+r+b)(w-r)<0
因为w+r+b>0，所以w-r<0，即r>w，成立
条件二：b>w+r，推不出r>w

alanuonuo · 发表于 2010-8-3 01:38:07

4. at least 是>=，你要注意她买了>10个paperback哦～

mimioy · 发表于 2010-8-3 01:39:09

谢谢！

请教大家四道prep的题! 麻烦大家了

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