Q2:

If 2 different representatives are to be selected at random from a group of 10 employees and if p is the probability that both representatives selected will be women, is p > ATH connecttype="rect" gradientshapeok="t" extrusionok="f">ATH>?

(1) More than of the 10 employees are women.

(2) The probability that both representatives selected will be men is less than .

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.

Q25:

If the sequence x₁, x_2, x_{3, …,} x_n, … is such that x₁ = 3 and x_n+1 = 2x_n – 1 for n ≥ 1, then x₂₀ – x₁₉ =

A. 2¹⁹

B. 2²⁰

C. 2²¹

D. 2²⁰ - 1

E. 2²¹ - 1

题1）我解了2个1元2次不等式。不过太慢了，想看看有没有快点好办法。

2）C(2,y) / C(2,10) < 1/10 => y^2 - y - 9 < 0 => y < (1 + sqr(37) ) / 2; y = 10 - x => x > 10 - ( 1 + sqr(37) ) / 2

（此前，用 C(2,x) / C(2,10) > 1/2 同理得出 x > 7。因为1)只能得出 x > 5，所以不充分） 2) 如果用 6 代 sqr(37)，才可以得出 x > 6.5，但实际 6 < sqr(37)，所以也不充分

题2）我认为是A。