Prep-T1-Q125 (DS)

[oneseven]

Q125
The sequence a1, a2, a3,..., an of n integers is such that ak=k if k is odd and ak=-ak-1 if k is even. Is the sum of the terms in the sequence positive?

(1) n is odd
(2) an is positive

Answer: D

不是很懂題目再說甚麼

[amazingslim]

Q125
The sequence a1, a2, a3,..., an of n integers is such that ak=k if k is odd and ak=-ak-1 if k is even. Is the sum of the terms in the sequence positive?

(1) n is odd
(2) an is positive

Answer: D

不是很懂題目再說甚麼

一個數列，當k奇數時，ak=k
當k偶數時，ak=-ak-1
所以a1=1,a2=-a1=-1
a3=3,a4=-3
以此類推
(1) n is odd
數列總和=第n個數(因為前面兩兩一組總和為0)，所以總和一定為正
(2) an is positive
第n個數為正，也就是n is odd, 結果跟(A)一樣，總和一定為正