下面几题都不会做，诚心请教!
Q16:
In the sequence of nonzero numbers t1, t2, t3, …, tn, …, tn+1 = tn / 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.
Q19:
Which of the following is equal to (212 – 26) / (26 – 23)?
A. 26 + 23
B. 26 - 23
C. 29
D. 23
E. 2
Q20:
Of the students in a certain school, 15 percent are enrolled in an art class and 10 percent are enrolled in a music class. What percent of the students in the school are enrolled in neither an art class nor a music class?
(1) 2/3 of the students who are enrolled in an art class are also enrolled in a music class.
(2) There are more than 100 students in the school.
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.
Q16
条件(1) t3=1/4 那么由于题目里面的 tn+1 = tn /2
可以知道t4 = 1/8 t5 = 1/16， SUFFICIENT
条件(2) t1-t5 = 15/16
同样 t5 = t4 /2 = t3 / 4 = t2 / 8 = t1 / 16
从而 t1 - t5 = t1 - t1 /16= 15/16 t1 = 15/16
从而t1 = 1 t5 = 1/16 SUFFICIENT
The correct answer is D
Each statement alone is sufficient
4-19这个考察的是因式分解。
(212 – 26) ＝ (26 – 23)（ 26 + 23）
E. Statements (1) and (2) TOGETHER are NOT sufficient.
（1）显然是单独充分的。
（2)根据题干t5=t4/2=2t3/2=8t1/2，与条件（2）可解出t5
所以选D
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