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Mathematica competitions in Croatia

Croatia geometry

Problem

For a real number , let be the parabola given by the equation . Prove that all parabolae pass through the same point.
Solution
Let us find a point that lies on all parabolae for any real .

The equation of is .

Let us try to find and such that for all , .

Rewriting: For this to be independent of , the coefficient of must be zero: So, for : Therefore, the point lies on all parabolae .

Thus, all parabolae pass through the same point .

Techniques

Cartesian coordinatesConstructions and loci