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China Mathematical Competition (Shaanxi)

China geometry

Problem

Let a line with the inclination angle of be drawn through the focus of the parabola . If the two intersection points of the line and the parabola are and , and the perpendicular bisector of the chord intersects the -axis at the point , then the length of the segment is ( ).

(A) (B) (C) (D)
Solution
It follows from the property of the focus of a parabola that . Then the equation of the straight line through points and will be . Substitute it into the parabola equation, and then obtain Let be the midpoint of the chord , then the -coordinate of is . Then we have , . Answer: A.
Final answer
A

Techniques

Cartesian coordinatesTrigonometryAngle chasing