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Geometry Difficulty 4.5 AIME Prove it Croatia

For a real number aa, let PaP_a be the parabola given by the equation y=x2+ax+(2014a)y = x^2 + a x + (2014 - a). Prove that all parabolae PaP_a pass through the same point.

Solution

Let us find a point (x0,y0)(x_0, y_0) that lies on all parabolae PaP_a for any real aa.

The equation of PaP_a is y=x2+ax+(2014a)y = x^2 + a x + (2014 - a).

Let us try to find x0x_0 and y0y_0 such that for all aa, y0=x02+ax0+(2014a)y_0 = x_0^2 + a x_0 + (2014 - a).

Rewriting:
y0=x02+ax0+2014a y_0 = x_0^2 + a x_0 + 2014 - a
y0=x02+2014+a(x01) y_0 = x_0^2 + 2014 + a(x_0 - 1)
For this to be independent of aa, the coefficient of aa must be zero:
x01=0    x0=1 x_0 - 1 = 0 \implies x_0 = 1
So, for x0=1x_0 = 1:
y0=(1)2+a(1)+2014a=1+a+2014a=1+2014=2015 y_0 = (1)^2 + a(1) + 2014 - a = 1 + a + 2014 - a = 1 + 2014 = 2015
Therefore, the point (1,2015)(1, 2015) lies on all parabolae PaP_a.

Thus, all parabolae PaP_a pass through the same point (1,2015)(1, 2015).

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