One or more options correct
Let be the point on the parabola , and let the focal chord through meet the parabola again at . Which of the following is (are) correct?
- A. The tangent and the normal at together with the -axis enclose a triangle of area
- B. The length of the focal chord is
- C. The normal at meets the parabola again at
- D. The circle on as diameter touches the line
Show answer and solution
Answer: B
Write with . The tangent is , meeting the -axis at ; the normal is , meeting the axis at . The triangle has base and height , so area (A false). The other end of the focal chord has parameter , i.e. , and the focal-chord length is (B true). The normal at parameter meets the curve again at , i.e. at , not (C false). A circle on any focal chord as diameter touches the directrix : centre , radius , so the leftmost point is , not (D false). Answer: B.
Saved on this device. Sign in or create a free account to keep it on every device.