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Algebra Difficulty 5.4 AIME, harder Prove it China

The number of real solutions for equation
(x2006+1)(1+x2+x4++x2004)=2006x2005 (x^{2006} + 1)(1 + x^2 + x^4 + \cdots + x^{2004}) = 2006x^{2005}
is .\underline{\quad}.

Solution

We have
(x2006+1)(1+x2+x4++x2004)=2006x2005(x+1x2005)(1+x2+x4++x2004)=2006x+x3+x5++x2005+1x2005+1x2003+1x2001++1x=20062006=x+1x+x3+1x3++x2005+1x20052×1003=2006, \begin{align*} & (x^{2006} + 1)(1 + x^2 + x^4 + \cdots + x^{2004}) = 2006x^{2005} \\ \Leftrightarrow & \left(x + \frac{1}{x^{2005}}\right)(1 + x^2 + x^4 + \cdots + x^{2004}) = 2006 \\ \Leftrightarrow & x + x^3 + x^5 + \cdots + x^{2005} + \frac{1}{x^{2005}} + \frac{1}{x^{2003}} + \frac{1}{x^{2001}} \\ & \qquad + \cdots + \frac{1}{x} = 2006 \\ \Leftrightarrow & 2006 = x + \frac{1}{x} + x^3 + \frac{1}{x^3} + \cdots + x^{2005} + \frac{1}{x^{2005}} \\ & \ge 2 \times 1003 = 2006, \end{align*}
where the equal holds if and only if x=1xx = \frac{1}{x}, x3=1x3x^3 = \frac{1}{x^3}, ...,
x2005=1x2005x^{2005} = \frac{1}{x^{2005}}. Then x=±1x = \pm 1.
Since x0x \le 0 does not satisfy the original equation, x=1x = 1 is
the only solution. So the number of real solutions is 1.

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