Friday, March 23, 2007

DS Question - 20

Series of A(n) is such that A(n) = A(n-1) / n. How many elements of the series are bigger than 1/2 ?

(1) A(2) = 5
(2) A(1) - A(2) = 5

Answer : D

From statement (1) --- A(2) = 5 . It is given that A(n) = A(n-1) / n.
Hence A(2) = A(1) / 2
=> A(1) = 10
A(2) = 5
=A(3) = A(2) / 3 = 5 /3
A(4) = A(3) / 4 = 5 /( 3 * 4) = 5 /12

Hence statement (1) alone is sufficient to aswer the question.

From statement (2) --- A(1) - A(2) = 5
=> A(1) - A(1) /2 = 5
=> A(1) = 10
Hence A(2) = 5
A(3) = A(2) / 3 = 5 /3 ...

Hence statement (2) alone is sufficient to answer the question.

NOTE: Always remember sum of series is

S_n=x_1+x_2+\dots + x_n=\sum\limits_{i=1}^{n}x_i.
Further n cannot be equal to zero or negative as n = number of the term and not the value of the term.