求助几道数学题的解法~谢谢亲们前来解答~

waterhouse · 发表于 2013-7-15 11:12:05

1 If 1050 – 74 is written as an integer in base 10 notation, what is the sum of the digits in that integer?
A. 424
B. 433
C. 440
D. 449
E. 467

  答案是C

2  Q23:
In the xy-plane, point (r, s) lies on a circle with center at the origin.  What is the value of r2 + s2?
(1)    The circle has radius 2.
(2)    The point (√2, -√2) lies on the circle.

A. Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient.
B. Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient.
C. BOTH statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient.
D. EACH statement ALONE is sufficient.
E. Statements (1) and (2) TOGETHER are NOT sufficient.
答案是D  我选了E  （2）只是说那个点在圆上，那圆的位置可以有好几个啊~怎么就能和（1）一起确定题干呢？

3 A photographer will arrange 6 people of 6 different heights for photograph by placing them in two rows of three so that each person in the first row is standing in f  ront of someone in the second row.  The heights of the people within each row must increase from left to right, and each person in the second row must be taller than the person standing in front of him or her.  How many such arrangements of the 6 people are possible?
A. 5
B. 6
C. 9
D. 24
E. 36
答案是A  我选了B  我是用列举法  如果6个人身高用1-6表示，则第二排第一排战法分别由： 456 123 256 134 356 124 256 134 246 135  346 125
有更简便的方法吗？我上面列举的有哪个不对吗？

4 If n = 811 – 8, what is the units digit of n?

A. 0
B. 1
C. 4
D. 6
E. 8
这种位数题要怎么做呢？答案是C

Joanna bought only $0.15 stamps and $0.29 stamps.  How many $0.15 stamps did she buy?
(1)    She bought $4.40 worth of stamps.
(2)    She bought an equal number of $0.15 stamps and $0.29 stamps.

A. Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient.
B. Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient.
C. BOTH statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient.
D. EACH statement ALONE is sufficient.
E. Statements (1) and (2) TOGETHER are NOT sufficient.
答案是1  可我觉得两个条件都用上 0.15X+0.29X=4.4  X 可以的出来啊~

A positive integer n is said to be “prime-saturated” if the product of all the different positive prime factors of n is less than the square root of n.  What is the greatest two-digit prime-saturated integer?

A. 99
B. 98
C. 97
D. 96
E. 95
答案是D  关键是" if the product of all the different positive prime factors “这个是指所有质数因子还是所有质数因子的和啊？

Q37:
A jar contains 16 marbles, of which 4 are red, 3 are blue, and the rest are yellow.  If 2 marbles are to be selected at random from the jar, one at a time without being replaced, what is the probability that the first marble selected will be red and the second marble selected will be blue?

A. 3/64
B. 1/20
C. 1/16
D. 1/12
E. 1/8
答案是B  我觉得不是4/16  *  3/12=1/16 吗？

谢谢各位~~

moreday · 发表于 2013-7-15 11:19:04

第一个题是10^50 - 74这样吧 =99999....926 所以是48*9+2+6=440

waterhouse · 发表于 2013-7-15 11:23:52

moreday 发表于 2013-7-15 11:19
第一个题是10^50 - 74这样吧 =99999....926 所以是48*9+2+6=440

哦！~谢谢~一开始我不理解那个in base 10 notation的意思~

2013GMAT · 发表于 2013-7-15 11:32:55

第三题你写了两遍256 134这个可能。最后一题应该是4/16*3/15 = 1/20

2013GMAT · 发表于 2013-7-15 11:36:38

第四题是8^11-8? 8得一次方个位是8，2次方个位是4。。以此类推个位的循环就是8，4，2，6，所以8的11次方的个位就是2。然后再用2减8就等于4了

waterhouse · 发表于 2013-7-15 11:38:20

2013GMAT 发表于 2013-7-15 11:32
第三题你写了两遍256 134这个可能。最后一题应该是4/16*3/15 = 1/20

哦对~我犯低级错误了~

2013GMAT · 发表于 2013-7-15 11:40:20

第二题圆的公式 r^2+s^2=半径的平方。1 告诉你了半径，2 告诉你了一点可以算出半径，所以就都sufficient。

waterhouse · 发表于 2013-7-15 11:40:37

2013GMAT 发表于 2013-7-15 11:36
第四题是8^11-8? 8得一次方个位是8，2次方个位是4。。以此类推个位的循环就是8，4，2，6，所以8的11次方的 ...

I know
~~thx~~~~

pisco · 发表于 2013-7-15 15:11:05

楼主做题一定要仔细看清楚题目啊。。。不然亏大了。。。
第二题：题目说了圆心在原点，所以（2）就有半径了。
第五题：我第一次做prep也被骗了，其实（1）以及题目的数值很巧的，x、y都要是整数。
式子化简是：15x+29y=440，15x的尾数必为5或0，要配合440的和，29y的尾数也要是5或0，所以y 的取值可以是5或10；再计算x，若y为5，则15x为295，x 不是整数；若y为10，x为10，刚刚好。所以（1）够了。
第六题：the product of all the different positive prime factors of n 是指“n所有不同质因子的乘积”，先把选项因数分解再乘一下就行了。一定是小于10的才可以选。

waterhouse · 发表于 2013-7-15 15:29:55

pisco 发表于 2013-7-15 15:11
楼主做题一定要仔细看清楚题目啊。。。不然亏大了。。。
第二题：题目说了圆心在原点，所以（2）就有半径了 ...

嗯〜这几题很多都是自己没看清题干导致的〜上午做题太浮躁〜thx〜

求助几道数学题的解法~谢谢亲们前来解答~

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