Hidden Property Trick of A Specific Function

Algebra Level pending

Given this function: f ( x ) = 2 2 + 4 x f(x) = \frac {2}{2+4^x}

Then calculate the value of: f ( 1 2014 ) + f ( 2 2014 ) + f ( 3 2014 ) + + f ( 2012 2014 ) + f ( 2013 2014 ) f(\frac {1}{2014}) + f(\frac {2}{2014}) + f(\frac {3}{2014}) + \ldots + f(\frac {2012}{2014}) + f(\frac {2013}{2014})

=> Write your answer in decimal form using a dot for the decimal point (not a coma.)


The answer is 1006.5.

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1 solution

Ariel Gershon
May 22, 2014

The trick here is that for a R a \in \mathbb{R} , we have f ( a ) + f ( 1 a ) = 1 f(a) + f(1-a) = 1 . Let's prove this:

Let a R a \in \mathbb{R} . Then, f ( a ) + f ( 1 a ) = 2 2 + 4 a + 2 2 + 4 1 a = 2 ( 2 + 4 1 a ) ( 2 + 4 a ) ( 2 + 4 1 a ) + 2 ( 2 + 4 a ) ( 2 + 4 1 a ) ( 2 + 4 a ) = 8 + 2 4 1 a + 2 4 a 8 + 2 4 1 a + 2 4 a = 1 f(a)+f(1-a) = \frac{2}{2+4^a} + \frac{2}{2+4^{1-a}} = \frac{2(2+4^{1-a})}{(2+4^a)(2+4^{1-a})} + \frac{2(2+4^a)}{(2+4^{1-a})(2+4^a)} = \frac{8+2*4^{1-a}+2*4^a}{8+2*4^{1-a}+2*4^a} = 1

Therefore, f ( 1 2014 ) + f ( 2013 2014 ) = 1 f(\frac{1}{2014}) + f(\frac{2013}{2014}) = 1 , f ( 2 2014 ) + f ( 2012 2014 ) = 1 f(\frac{2}{2014}) + f(\frac{2012}{2014}) = 1 , and so on. There are 1006 such pairs which together add to 1006. Then, the remaining term is \(f(\frac{1007}{2014}). This can be simplified:

\(f(\frac{1007}{2014}) = f(\frac{1}{2}) = \frac{2}{2+4^{1/2}} = \frac{1}{2}\)

Hence, the total sum is 1006.5.

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