数学求助。。。不胜感激！！！！

30453952 · 发表于 2010-10-23 21:36:50

If n is the product of the integers from1 to 20 inclusive, what is the greatest integer k for which 2^k is a factor of n?

A.10
B.12
C.15
D.18
E.20 答案D

A set of numbers has the property that for any number t in the set, t + 2 is in the set.If –1 is in the set, which of the following must also be in the set?

I.-3
II.1
III.5

A.I only
B.II only
C.I and II only
D.II and III only
E.I, II, and III
答案：D

For any positive integer n, the sum of the first n positive integers equals [n(n+1)]/2.What is the sum of all the even integers between 99 and 301?

A.10,100
B.20,200
C.22,650
D.40,200
E.45,150
答案：B

For any positive integer n, the sum of the first n positive integers equals [n(n+1)]/2.What is the sum of all the even integers between 99 and 301?

A.10,100
B.20,200
C.22,650
D.40,200
E.45,150
答案：B

30453952 · 发表于 2010-10-23 21:48:36

的确是不会啊求助求助

magicstar · 发表于 2010-10-23 21:52:10

第一题，我没有好办法，只是数偶数的2的次方了。
第二题,t+2,当t=-1,可推出3，5. 不可逆推，即不能用t-2推。
第三题，[（301×302）/2-（99×100）/2 ]/2约等于(300×300-100×100)/2,找到B。

笨笨熊41 · 发表于 2010-10-23 21:53:13

A set of numbers has the property that for any number t in the set, t + 2 is in the set.If –1 is in the set, which of the following must also be in the set?

I.-3
II.1
III.5

A.I only

B.II only
C.I and II only

D.II and III only

E.I, II, and III

答案：D
这个题目中要理解题目的意思，只可以从t=-1向后推，因为题目没有说明反之亦然，所以推不出（1）成立，只有（2）（3）可以正推出来，选D
For any positive integer n, the sum of the first n positive integers equals [n(n+1)]/2.What is the sum of all the even integers between 99 and 301?

A.10,100
B.20,200
C.22,650
D.40,200
E.45,150
答案：B
这道题中从99到301中有(（300-100）/2)+1=101个偶数，而且是等差数列平均数为（300+100）/2=200,sum=200*101

zouliwei · 发表于 2010-10-23 22:07:00

If n is the product of the integers from1 to 20 inclusive, what is the greatest integer k for which 2k is a factor of n?
找出1*2*3...*19*20的乘积里面有多少个质因子2
2，4，6，8，10，12，14，16，18，20 共有18个质因子2
所以k最大可以是18

A set of numbers has the property that for any number t in the set, t + 2 is in the set.If –1 is in the set, which of the following must also be in the set?
从t开始的等差数列，间距为2，又题目给定一个a，推出a+2可以，但是不能推出a-2可以，因为a可能是数列的第一个数
所以只有比-1大的可以选

For any positive integer n, the sum of the first n positive integers equals [n(n+1)]/2.What is the sum of all the even integers between 99 and 301?
就是求100-300inclusive的偶数和，等差数列间距2，一共101个数（102-300之间是100个偶数，这里还多一个100）
和=（100+300）*101/2=20,200

数学求助。。。不胜感激！！！！

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