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Algebra Difficulty 5.6 AIME, harder Prove it Ukraine

Solve the equation [x2]2x+1=0[x^2] - 2x + 1 = 0, where [x2][x^2] denotes the greatest integer that does not exceed x2x^2.

Solution

We have x=[x2]+12x = \frac{[x^2] + 1}{2}. Hence, x=tx = t, or x=t+12x = t + \frac{1}{2}, where tt is an integer.

If x=tx = t, we have
[x2]2x+1=[t2]2t+1=t22t+1=(t1)2=0t=1x=1. [x^2] - 2x + 1 = [t^2] - 2t + 1 = t^2 - 2t + 1 = (t - 1)^2 = 0 \Leftrightarrow t = 1 \Leftrightarrow x = 1.

If x=t+12x = t + \frac{1}{2}, then
[x2]2x+1=[(t+12)2]2(t+12)+1=[t2+t+14]2t1+1. [x^2] - 2x + 1 = [(t + \frac{1}{2})^2] - 2(t + \frac{1}{2}) + 1 = [t^2 + t + \frac{1}{4}] - 2t - 1 + 1.
Since t2+t+14t^2 + t + \frac{1}{4} is not an integer, [t2+t+14]=t2+t[t^2 + t + \frac{1}{4}] = t^2 + t.
So,
[x2]2x+1=t2+t2t1+1=t2t. [x^2] - 2x + 1 = t^2 + t - 2t - 1 + 1 = t^2 - t.
Thus,
t2t=0t=0 or t=1x=12 or x=32. t^2 - t = 0 \Leftrightarrow t = 0 \text{ or } t = 1 \Leftrightarrow x = \frac{1}{2} \text{ or } x = \frac{3}{2}.

Therefore, the solutions are x=1x = 1, x=12x = \frac{1}{2}, and x=32x = \frac{3}{2}.

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