555做了GMATPRE的模拟题目，伤心/几道题目请教

1.有限consecutive数列，number of variation in sign is defined as number of paris of consecutive terms of the sequence for which the product of the two consecutive terms is negative. what is the number of variation in sign for the sequence 1, -3,2,5,-4,-6?

A.1,  B2.      C,3,        D4 ,              E,5

ANSER:C---什么意思呢？题目

2. DS:On J's credit card account, the average daily balance for 30-day cycle is the average of daily balances at the end of each 30-day. at the begining, J saved $600, during the period, made a payment $300, 如果没有其他费用增减，what was the average daily balance on J's account for the billing cycle?

1) J payment is on the 21th day. 2) the avarage daily balance through 25th day was $540

答案：D---什么意思？怎么算？2）知道可以，但是1怎么算呢？

3.一条线围住成圆圈为2miles， J 要走完一次平均要3m/小时的速度才可，t表示J 走的时间，以下哪个是正确？

A,0.5< t< 0.75，B,1.75 <t< 2.0， C,2.0<t<  2.5，D,2.5<t<  3.0，E,3 <t< 3.5

答案：C。。。。什么呀？不是A么？怎么算呢？

4.If r and s are integer, rs+r is odd, what is even in the followings?

r,      s,      r+s,        rs-t,          r^2+s  不是C么？怎么答案是B？ r（s+1）是odd,那么r 和s同质，两个都是奇或者两个都是偶。那r+s一定是even呀，怎么是S？难道这个peason的软件答案有问题？

5.DS:在坐标里，a,b  是正数，CD的中点是？ 1）C点（a, 1-b)  2)D点(1-a, b)

答案是C，但是我觉得两个条件都算不出来啊。。。。请教呀

谢谢

1.有限consecutive数列，number of variation in sign
        is defined as
        number of paris of consecutive terms of the sequence for which the product of the two consecutive terms is negative. what is the number of variation in sign for the sequence 1, -3,2,5,-4,-6?

A.1,  B2.      C,3,        D4 ,        E,5

题目就是说“number of variation in sign”的定义是序列中两个连续的乘积为负的数，问有几个这样的东东。很显然1跟－3，－3跟2，5跟－4符合这个定义，所以答案有三个，选c

1) J payment is on the 21th day. 2) the avarage daily balance through 25th day was $540

这道题OG黄色那本上有解释，题号是141题.

3.一条线围住成圆圈为2miles， J 要走完一次平均要3m/小时的速度才可，t表示J 走的时间，以下哪个是正确？

A,0.5< t< 0.75，B,1.75 <t< 2.0， C,2.0<t<  2.5，D,2.5<t<  3.0，E,3 <t< 3.5

这题我也不太清楚@.@

4.If r and s are integer, rs+r is odd, what is even in the followings?

r,      s,      r+s,        rs-t,          r^2+s 

这题其实变变形就很简单了rs+r=r(s+1),积为奇，奇×奇＝奇，所以s为偶

5.DS:在坐标里，a,b  是正数，CD的中点是？ 1）C点（a, 1-b)  2)D点(1-a, b)

解：线段终点坐标为((x1+x2)/2,(y1+y2)/2),所以给了两个点的坐标就能得出中点坐标啦：），中点坐标为((a＋(1-a))/2,

(1-b+b)/2),即（1/2，1/2）

加油噢！！：）

不是很明白：（

两个连续的乘积为负，1跟－3，－3跟2，5跟－4怎么符合这定义？都不是连续的啊，我们排列大小：-6，-4，-3，1，2，5....是负数的，而且连续的不就只有-3和1么？

按照你那样说的话，既然5和-4都是，为什么5和-6不是？不明白不明白

鼓掌：）