[求助]四题Prep数学

purplegosh

想问这四题数学的解法

都是PP2里面破解版没有详解的

因为今天模考把题目跟答案抄下来

数学本身程度不好

希望有人可以帮忙一下

谢谢!!!

41.    5689-!-item-!-187;#058&004421

In the infinite sequence a1, a2, a3, . . . , an, . . . , each term after the first is equal to twice the previous term.  If a5 - a2 = 12, what is the value of a1 ?

 

(A) 4

(B) 24/7

(C) 2

(D) 12/7

(E) 6/7

答案:E

77.    8876-!-item-!-187;#058&006751

A contest will consist of n questions, each of which is to be answered either "True" or "False." Anyone who answers all n questions correctly will be a winner.  What is the least value of n for which the probability is less than 1/1000 that a person who randomly guesses the answer to each question will be a winner?

 

(A)  5

(B)  10

(C)  50

(D)  100

(E) 1,000

答案:B

135.  13220-!-item-!-187;#058&009935

Machine A produces pencils at a constant rate of 9,000 pencils per hour, and machine B produces pencils at a constant rate of 7,000 pencils per hour.  If the two machines together must produce 100,000 pencils and if each machine can operate for at most 8 hours, what is the least amount of time, in hours, that machine B must operate?

 

(A) 4

(B)  4又2/3

(C)  5又1/3

(D) 6

(E)  6又1/4

答案:A

165.  17668-!-item-!-187;#058&011589

A circular cloth with a diameter of 20 inches is placed on a circular tray with a diameter of 24 inches.  What fraction of the tray's surface is not covered by the cloth?

 

(A)  1/6

(B)  1/5

(C)  11/36

(D)  25/36

(E)  5/6

答案:C

bolide

1 An=(An-1)*2^(n-1)
2.(1/2)^n<=1/1000
3.B=(10000-9000*8)/7000
4.圆面积差/小圆面积

purplegosh

谢谢bolide喔!!真厉害

不过我带入算了一遍

第一题还是算不出答案....

然后第四题我算出来的答案是11/25

结果没这个答案耶是我哪里算错了吗

wawadream

a5-a2=a1(2^4-2)=14a1=12=>a1=12/14=6/7,mm只要细心就好啦！

purplegosh

谢谢wawadream喔!

数学还真的是要细心呢

希望Q有49我就要偷笑了