quizicalindian
If f(x) = f(x - 1) + f(x - 2) for all x > 2; is f(5) a perfect square ?
(1) f(2) = 2 f(1)
(2) The first two terms of the series, f(1) and f(2) are consecutive even numbers
\(f(5) = f(4)+f(3).\)..(i)
\(f(4) = f(3)+f(2)...(ii)\)
\(f(3) = f(2)+ f(1)...(iii)\)
\(f(5) = f(2)+ f(1) +f(2) +f(2)+ f(1)...\) putting (ii) and (iii) in (i)
Thus
\(f(5) = 3f(2) +2f(1)... (iv)\) Thus if we can find \(f(2)\)mand \(f(1)\) we can answer the stem.
(1) f(2) = 2 f(1)Using \((iv)\) we have \(6f(1)+2f(1) = 8f(1)\)
However we do not know \(f(1)\)
INSUFF.(2) The first two terms of the series, f(1) and f(2) are consecutive even numbers.We could have \(f(1)\) and \(f(2)\) with many possibilities such as:
\(2\) and \( 4 \)
\(0\) and \( 2 \)
\(0 \) and \(-2 \)
\(-2\) and \(-4 \)
etc.
As such we will get many values for \(f(5) = 3f(2) +2f(1)\)
INSUFF.1+2
\(f(1)=-2 \) and \( f(2)=-4\) or:
\(f(1)=\hspace{1mm}2 \hspace{1mm}\)and \(\hspace{3mm} f(2)=\hspace{1mm}4 \)
Thus using eqn.(iv) we can get \(f(5)= 16\) or \(f(5)= -16 \)
Hence we can get both a
YES and a
NO. IMO ans should be E
However if the author intended only +ve integers as consecutive even integers then \(f(1)=2 \) and \(f(2)=4\) then \(f(5) =16\) and ans would be C.
Hope it helps.
quizicalindian, please can you check with the author of the question, and clarify. Thanks.