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The solution of the recursive relation 
for n ≥ 2, a0 = 1, a1 = 0
image:EM4/Q10a.PNG

  1.  image:EM4/O10a.PNG
  2.   image:EM4/O10b.PNG

     
  3.   image:EM4/O10c.PNG
  4.   image:EM4/O10d.PNG
asked in Combinatory by Veteran (12.6k points)   | 134 views

2 Answers

+3 votes
Best answer
Put n = 1 we get $\left ( \frac{1}{2} \right )^{3} - \left ( \frac{-1}{2} \right )^{3} = \left ( \frac{1}{8} \right ) + \left ( \frac{1}{8} \right ) = \left ( \frac{1}{4} \right )$ which is matched with given reccurence relation .= 1/4

Put n= 2 we get $\left ( \frac{1}{2} \right )^{4} - \left ( \frac{-1}{2} \right )^{4} = \left ( \frac{1}{16} \right ) - \left ( \frac{1}{16} \right ) = \left ( 0 \right )$  which is matched with given reccurence relation .= 0

Put n= 3 we get $\left ( \frac{1}{2} \right )^{5} - \left ( \frac{-1}{2} \right )^{5} = \left ( \frac{1}{32} \right ) + \left ( \frac{1}{32} \right ) = \left ( \frac{1}{16} \right )$  which is matched with given reccurence relation .= (1/4 )/4 = (1/16)

So B is answer
answered by Veteran (44.8k points)  
selected by

by putting n=2 ,we will get 1/4 as a0 =1

and answer given is B actually
now check always try to put in these quetion :) but no wrongly like i did :)
thanks a lot @anirudh..
+1 vote

4 an - an-2 = 0 ...............from the given recurrance

Now, adding and subtracting an-3,

\therefore  4 an - an-2 + an-3 - an-3 = 0

\therefore  4 an - an-2 + an-3 4 an-1 = 0      (from the recurrance for an-1)

\therefore   4 an 4 an-1 - an-2 + an-3= 0  

This is  our normal recurrance now.

\therefore 4t3 - 4t2 - t +1 =0

By trial and error, 1/2 is one of the factors.

Now, we perform synthetic division.

1/2  |     4    -4       -1      1

        |          +2       -1      -1

---------------------------------------

            4     -2       -2       0

So, factorisation is

(t-1/2)(t -1)( 2t +1) = 0

So, factors are t= 1/2 , -1/2, 1

So, f(n) = c1 . (1/2)n + c2. (1)n + c3. (-1/2)n

 

Now, putting the values of a0 and a1,   we get answer B.

answered by Veteran (14.8k points)  


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