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f(f(x)) = 13 - 2(f(x))
Decide f(f(x))
 Jan 29, 2014
 #1
avatar+2353 
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Werter_380:

f(f(x)) = 13 - 2(f(x))
Decide f(f(x))



Sorry, but without a single solution such as for example f(0) = 1 this cannot be written in another way.

Perhaps I like Serena or Melody know something different then me about this, but it might take a while before they look at this.
 Jan 29, 2014
 #2
avatar+117845 
0
Thanks reinout-g
I actually looked probably when you did.
I've got no good ideas.
I don't think it is solvable but i could be wrong.
Sorry.
 Jan 30, 2014
 #3
avatar+135 
0
Hey Werter!

I see someone referred to me. Suggesting that I may have a solution.
Well... I do not have a solution either.

Buuuuut... let's see what we can say...
There is more than 1 solution, so we need indeed some extra condition.

Suppose for instance that f(x)=c for some constant c.
Then the equation becomes:

f(f(x)) = 13 - 2 f(x)
f(c) = 13 - 2 c
c = 13 - 2c
3c = 13
c = 13 / 3

So the corresponding solution is f(x) = 13 / 3.

Or suppose f(x) = ax + b with non-zero a.
Then:

f(f(x)) = 13 - 2 f(x)
f(ax + b) = 13 - 2 (ax + b)
a (ax + b) + b = 13 - 2 (ax + b)
(a 2 + 2a) x = 13 - ab - b - 2b ................. using Melody's hint to use [sup]
a (a + 2) x = 13 - (a + 3) b

Since this equation should hold for any x, both the left side and the right side will have to be zero.
Since we started with a non-zero "a", we need a = -2 and b = 13.
So another solution is f(x) = -2x + 13.
 Jan 30, 2014
 #4
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0
u know, sry but i dont know lots about math.... sorry!!
 Jan 30, 2014

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