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Functional equations

  1. Feb 21, 2010 #1
    Hello,

    could you explain me what's the right way to solve these equations. i've never solved it before.

    [tex]f(x+y)+f(x-y)=2f(x)f(y)\,\;\;\forall x,y\in\mathbb{R}[/tex]
    [tex]f(x)+\left(x+\frac{1}{2}\right)f(1-x)=1\,\;\;\forall x\in\mathbb{R}[/tex]

    thank you............
     
  2. jcsd
  3. Feb 21, 2010 #2
    Since the equation is true for all values, use clever values of x and y to get a system of equations.
     
  4. Feb 21, 2010 #3

    HallsofIvy

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    Science Advisor

    For example, if, in the first, we take y= 0, we get f(x+0)+ f(x- 0)= 2f(x)= 2f(x)f(0) so we must have f(0)= 1. But if we take x= 0, we get f(0+y)+ f(0- y)= f(y)+ f(-y)= 2f(0)f(y) = 2f(y). That is, f(-y)= f(y) for all y so f is an even function.
     
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