pp数学求救

chuyou1989

1，If x and y are positive integers such that x = 8y + 12, what is the greatest common divisor of x and y ?

(1) x = 12u, where u is an integer.

(2) y = 12z, where z is an integer.

答案是B
用（2）求出的最大公约数是12吗？
（1）的话最后求出y=3/2（u-1）,因此y与x的一个公约数为3，但是(u-1)/2与 4u无法判断是否有还有公约数，因此无法判断吗？

2，At a certain supplier, a machine of type A costs $20,000 and a machine of type B costs $50,000.Each machine can be purchased by making a 20 percent down payment and repaying the remainder of the cost and the finance charges over a period of time.If the finance charges are equal to 40 percent of the remainder of the cost, how much less would 2 machines of type A cost than 1 machine of type B under this arrangement?



(A) $10,000

(B) $11,200

(C) $12,000

(D) $12,800

(E) $13,200

这道题完全看不懂，能够帮忙解释一下什么意思吗？

philover

第二题是说，20000块买个机器的话，可以选择分期付款，这样首付80%。剩下20%过段时间再付，但是需要抽取这延期付款部分的40%作为利息。就是用这种方式购买20000的机器的话，先付16000，剩下的4000过段时间再付，但是需要追加1600的利息

yydetiankong

怎么第二题算下来10800？？？帮顶~

BlackTTea

额~我理解的是首付20%，剩下的80%可以以后付。我算出来是E，不知道对不对。。。

chuyou1989

LS的同学对了
请帮忙讲下吧
怎样能算出是E呢？

chuyou1989

明白了
是E
哪位大牛能帮忙讲下第一题？

yexuu

第一题，第一个条件可以用举反例排除，第二个条件y=12z，那么x=12(8z+1),所以就要关注8z+1和z有没有公约数，假设他们的公约数是a（a》1），那么8Z+1=ab,z=ac,so 8ac+1=ab,then a(8c-b)+1=0, given a,b,c are all positive intergers, it is imposible for this equality to be correct. 所以最大公约数是12，因此选b
第二题down payment的意思是首付款，其它翻译见二楼，答案是E

chuyou1989

谢谢LS的同学
明白了

BlackTTea

2 machines of type A：20,000*2*20%+20,000*2*80%*(40%+1)=52,800
1 machine of type B: 50,000*20%+50,000*80%*(1+40%)=66,000
66,000-52,800=13,200

chuyou1989

谢谢楼上几位同学
数学小白在求助一道题
关于单利与复利的
Bob invested $2,000 in fund A and $1,000 in fund B.  Over the next two years, the money in fund A earned a total interest of 12 percent for the two years combined and the money in fund B earned 30 percent annual interest compounded annually.  Two years after Bob made these investments, Bob's investment in fund A was worth how much more than his investment in fund B ?

(A) $500
(B) $550
(C) $600
(D) $650
(E) $700
答案是B
怎样算出的呢？the money in fund A earned a total interest of 12 percent for the two years combined and the money in fund B earned 30 percent annual interest compounded annually。这句话是什么意思啊？