救命！4数学－GWD13套第三套－Q6, 17, 19, 31

zsx

    

Q6.
A box contains 10 light bulbs, fewer than half of which are defective.
2 bulbs are to be drawn simultaneously from the box. If n of the bulbs
in a box are defective, what is the value of n?

(1) The probability that the 2 bulbs to be drawn will be defective is 1/15.

(2) The probability that one of the bulbs to be drawn will be defective and the other will not be defective is 7/15.

        

The answer is D. 我相信“从盒子里同时拿出两个灯泡都是defective的概率” 和
”从盒子里拿出第一个灯泡是defective，在不添加新灯泡的情况下，拿出的第二个灯泡也是defective的概率“
是不一样的。可是应该如何表示 “同时取出的两个灯泡都是defective” 的概率？难道是 (n/10) x 2? 好像怪怪的。没有办法想通
... HELP!!

        

Q17. If a certain coin is flipped, the probability that the coin will
land heads is 1/2. If the coin is flipped 5 times, what is the
probability that it will land heads up on the FIRST 3 flips and not on
the last 2 flips.

A. 3/5

B. 1/2

C. 1/5

D. 1/8

E. 1/32

        

对这一题，试图想过几遍，可是就完全没有办法在 first 3 flips and last 2 flips 这里想通。 急需救援！！！

答案给出的是E。

        

        

Q31. A small, rectangular park has a perimeter of 560 feet and a
diagonal measurement of 200 feet. What is the are, in square feet?

A. 19,200

B. 19,600

C. 20,000

D. 20,400

E. 20,800

        

我以为这是一个稍微简单一点的题目。可是我得到的答案是9,600。 我的想法：2 x (L + W) = 560 ---> L + W =
280; L^2 + W^2 = 200^2; (L + W)^2 = L^2 + W^2 + 2 x L x W = 280^2
---> L x W = 9,600. 是哪里算错了吗？还是我理解出错了？

        

虽然题目可能笨了点，可是我就是比较困惑；不知如何破解。请大家帮忙解答一下。感激！！！

zl35

1。同时取出的两个灯泡都是defective的概率=Cn2/C102

2.就是说前三次是正面，后两次是反面。再用乘法原理

3，你算错了

zsx

感谢！！！