functions

If

f(x) = \frac{1}{3}[f(x) + \frac{5}{f(x+2)}]

then \lim_{x\rightarrow \infty }f(x) = ?

edit: f(x) is not a constant function and f(x) > 0 in its domain

10 Answers

341
Hari Shankar ·

have you typed it correctly? f(x) appears on both sides

106
Asish Mahapatra ·

yeah the expression had f(x) on both sides.

btw this is not my dbt :)

11
Tush Watts ·

Is the ans is √ (5/2) ??

106
Asish Mahapatra ·

no .. think a little more

11
Tush Watts ·

@ Asish

Since lim x→∞ f(x) = lim x→∞ f(x) = lim x→∞ f(x+2) = a (say)
then a = (1/3) (a + 5/a )
THerefore 2 a = 5 / a therefore, a = √(5/2)
Hence lim x→∞ f(x) = √(5/2)

106
Asish Mahapatra ·

no ...

analyse the givn condition a bit more . see if u can get a result from it which would automatically reject this answer

btw if the exp was f(x) = 13[f(x+1) + 5f(x+2)

then ur answer wud have been correct

24
eureka123 ·

hey one thing I noticed..this function is periodic with period 4...
I know this has no connection with the ques though :P

106
Asish Mahapatra ·

I know this has no connection with the ques though :P

why so??

this was the thing i expected Tushar to notice and it has a strong bearing with the answer

24
eureka123 ·

really ??????
looks like I am near the answer then...
actually i took this observation very lighlty[3]

106
Asish Mahapatra ·

see:
as the function is periodic,
f(x) = f(x+4)

So when x→∞, u cant say that f(x) = f(x+2) (as the function is periodic)

So, the answer is

Cant Say..

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