求助：几道新下载GMAT prep数学题

zoro513

#11. If n and y are positive integers, and 450y=n³ , which of the following must be an integer?

I. y/(3x2²x5)

II. y/(3²x2x5)

III. y/(3x2x5²)

A) None B) I only C)II only D) III only E) I, II and III

答案是 B.  不懂。

#16. for how many integers n is 2ⁿ = n² ?

答案是 "2".   "for how many integer" 是不是指有几个？可为什么有两个？

#20 Data Sufficiency: if x and y are integers greater than 1, is X multiple of Y?

1) 3y² + 7y = x

2) x² - x is a multiple of y

答案是 A.   我实在想不通

#33 Data sufficiency: how many odd integers  are greater than integer x and less than integer y?

1) There are 12 even integers greater than x  and less than y

2) there are 24 integers greater than x  and less than y

答案是 B. 为什么1）不行？

bigbighead

#11. If n and y are positive integers, and 450y=n³ , which of the following must be an integer?

I. y/(3x2²x5)

II. y/(3²x2x5)

III. y/(3x2x5²)

A) None B) I only C)II only D) III only E) I, II and III

答案是 B.  不懂。 把450分解了＝2*3^2*5^2  那么要保证450y是n^3  Y 至少是 2^2*3*5  所以1 正确 其他都不对

#16. for how many integers n is 2ⁿ = n² ?

答案是 "2".   "for how many integer" 是不是指有几个？可为什么有两个？画图 一个是抛物线 一条是递增曲线 有两个交点， 所以是2 我做这题时就这么想的

#20 Data Sufficiency: if x and y are integers greater than 1, is X multiple of Y?

1) 3y² + 7y = x

2) x² - x is a multiple of y

答案是 A.   我实在想不通 1） y(3y+7)/y= 3y+7 y是 非零整数 正确 2） 举个反例 x＝4 y=6 4不是6的倍数

#33 Data sufficiency: how many odd integers  are greater than integer x and less than integer y?

1) There are 12 even integers greater than x  and less than y

2) there are 24 integers greater than x  and less than y

答案是 B. 为什么1）不行？1不行啊 因为题目没有告诉 x 和y 是 偶数还是奇数 如果两个都是偶数 那么他们之间的奇数是13个 如果x y都是奇数就只有11个 还有 1奇 1偶的情况。。。。

moo_moo_cow

11. 450y = 3^2*5^2*2*y = n^3.

      let y/(3*2^2*5) = k therefore y = 3*2^2*5*k

      subs into y and n relationship, 3^3*5^3*2^3*k = n^3

      (3*5*2*a)^3 = n^3 therefore n = 3*5*2*a = integer

      Since 2,3 and 5 are integers, therefore a must be integer hence k must be integer since k = a^3.

16. "for how many integers" means how many integers fulfill this relationship of 2^n = n^2

       the answer is 2 integers i.e 2 and 4 because when n > 5, 2^n will definitely greater than n^2.

20. X = kY

     (1)y(3y+1) = x = ky, thus (1) alone is sufficient.

     (2)x(x-1) = ky, it can be x or (x-1) is multiple of y thus (2) is insufficient.

33. (1) is insufficient because integer x can be odd or even and integer y can be odd or even as well. Multiple 

      possibilities.

      (2) indicates 24 integers between x and y thus x is same as y => odd or even i.e 12 odd integers.

zoro513

太谢谢了。