數學求解~~~~~(急)

戚玥

1.    If each term in the sum a1+a2+......+an is either 7or77 , and the sum equals 350, which of the following could be equal to n?
那个sum是7的倍数 7（1X+11Y）=350
即   X+11Y=50   Y=1时X=39 n=40; Y=2时X=28 n=30; Y=3时X=17 n=20; Y=4时X=6 n=10.


2. mx+ky>kx+my?

 (1) m>k
 (2) x>y

答案: C
移项相消： mx+ky-kx-my=(m-k)x+(k-m)y=(m-k)(x-y)
肯定要看MK哪个大，XY哪个大~所以咯
1 不行
2 不行
1+2：可以


3.  A company has two types of machines, type R and S, a machine of type R does a certain job in 36 hours and machine of type S does the same job in 18 hours. If the company used the same number of each type of machine to do the job in 2 hous. how many machines of type R were used?
列式：1/【 （1/36+1/18）X 】=2
   得 X=6


4.on her way home from work, John drives through several tollbooths过路收费亭. Is there a pair of these tollbooths that are less than 10 miles apart?
  (1) the first tollbooth and the last are 25 apart
  (2) John drives through 4 tollbooths
1不行
2不行
1+2 总长度25 一共经过4个 25/4=六点多吧~小于10
   可以


5.  what was Jean's insurance premium in 1995?
  (1) the ratio of Jean's insurance premium in 1995 to her insurance premium in 1994 was 6/5
  (2) Jean's insurance premium in 1995 was 20% more than her  insurance premium  in 1994
1不行
2不行
1+2： X/Y=6/5
     1.2X=Y
两个式子一样的……也不行~

gilgemash

第1题根据题意1）7x+77y=350 2）x+y=n 求得n+10y=50 y=1,2,3,4 得n=40，30，20，10
第4题1）2）单独都INSUF，1+2得一共四个收费口，分3段，总长25m，3段中必然有一段即两个收费口的距离<10m SUF
第5题的两个条件是等价条件，只能退出94与95年保险费之比，无法得出具体数字

chasechase12

感謝各位, OK了!

gkast

提lz一下: 看到像第3题的形式，都可以马上这么解:

(1/R)X  + (1/S)X = 1 / result

lastjuly

thx~