GWD数学几道请教

夏日飘雪

1.In the sequence of nonzero numbers t₁, t₂, t₃, …, t_n, …, t_n+1 = t_n / 2 for all positive integers n.What is the value of t₅?
(1)t₃ = 1/4
(2)t₁ - t₅ = 15/16

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.

答案是选D，请问以上两个条件如何分别求出t5？非常感谢。。。。

2.A certain club has 20 members.What is the ratio of the member of 5-member committees that can be formed from the members of the club to the number of 4-member committees that can be formed from the members of the club?


A.16 to 1
B.15 to 1
C.16 to 5
D.15 to 6

E.5 to  4

这题为什么选C？主要是我题目看不大懂。。

3.If n is a positive integer and r is the remainder when (n – 1)(n + 1) is divided by 24, what is the value of r?
(1)2 is not a factor of n.
(2)3 is not a factor of n.

这题不会。。。请教NN。。。。帮忙解答下，O(∩_∩)O谢谢！！这题选C，但是我不知道思路怎么做。。。

ypl214

第一个 当然可以 就是一个公比为1/2的等比数列，再加一个随便条件就可以
第二个c(20,5)/c(20,4)   画出来就c
就是问抽出来5个人的组合数和抽出来4个人的组合数的比例
第三个我就是把数带进去算了算