题目没看懂。。

southnorth

请高手帮忙看看下面几题，我怎么都没想明白。

If n is a positive integer, what is the value of the hundreds digit of 30ⁿ

1.30ⁿ>1000

2. n is a multiple of 3

答案是两个都sufficient

For any integer k greater than 1, the symbol k* denotes the product of all the fractions of the form 1/t, where t is an integer between 1 and k, inclusive.  What is the value of 5*/4*

答案是1/5

What is the greatest common factor of the positive integers j and k?

1.k=j+1    2. jk is divisible by 5

答案是1成立，2废掉

Is |x|+|x-1|=1?

1.x>=0   2. x<=1

答案是合起来成立

[此贴子已经被作者于2009-1-21 10:24:22编辑过]

Oia

If n is a positive integer, what is the value of the hundreds digit of 30ⁿ

1.30ⁿ>1000

2. n is a multiple of 3

解：

对于1）可以推出，n>=3, 则hundreds digit=0， 单独充分；

对于2）可以推出，n=3,6,9。。。。则hundreds digit=0, 单独充分；

答案是两个都sufficient

For any integer k greater than 1, the symbol k* denotes the product of all the fractions of the form 1/t, where t is an integer between 1 and k, inclusive.  What is the value of 5*/4*

解：

由题意：k*= 1/1 * 1/2 * 1/3 ......*1/K

5*= 1/1 * 1/2 .....*1/5

4*= 1/1 * 1/2 .....1/4

答案是1/5

What is the greatest common factor of the positive integers j and k?

1.k=j+1    2. jk is divisible by 5

对于1），j与j+1两个正整数是互质的。最大公约数是1.

答案是1成立，2废掉

Is |x|+|x-1|=1?

1.x>=0   2. x<=1

对于1）， X>=0, 若X>1，则等式不成立，条件1不充分；

对于2）， X<=1， 在X<0 时，等式不成立，条件2不充分；

联立则充分。

答案是合起来成立