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JEE Advanced 2018 Paper 2 · Mathematics · Conic Sections · medium

Let P be the point (8,8) on the parabola y^2=8x, and let the focal chord through P meet the parabola again at Q.

One or more options correct

Let PP be the point (8,8)(8,8) on the parabola y2=8xy^{2}=8x, and let the focal chord through PP meet the parabola again at QQ. Which of the following is (are) correct?
  1. A. The tangent and the normal at PP together with the xx-axis enclose a triangle of area 4040
  2. B. The length of the focal chord PQPQ is 252\dfrac{25}{2}
  3. C. The normal at PP meets the parabola again at (32,−16)(32,-16)
  4. D. The circle on PQPQ as diameter touches the line x=−1x=-1
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Answer: B

Write P=(2t2,4t)P=(2t^{2},4t) with t=2t=2. The tangent ty=x+2t2ty=x+2t^{2} is 2y=x+82y=x+8, meeting the xx-axis at (−8,0)(-8,0); the normal y=−tx+4t+2t3y=-tx+4t+2t^{3} is y=−2x+24y=-2x+24, meeting the axis at (12,0)(12,0). The triangle has base 2020 and height 88, so area =80≠40=80\neq40 (A false). The other end of the focal chord has parameter −1t=−12-\frac1t=-\frac12, i.e. Q=(12,−2)Q=\left(\frac12,-2\right), and the focal-chord length is xP+xQ+2a=8+12+4=252x_P+x_Q+2a=8+\frac12+4=\frac{25}{2} (B true). The normal at parameter tt meets the curve again at t′=−t−2t=−3t'=-t-\frac2t=-3, i.e. at (18,−12)(18,-12), not (32,−16)(32,-16) (C false). A circle on any focal chord as diameter touches the directrix x=−2x=-2: centre x=174x=\frac{17}{4}, radius 254\frac{25}{4}, so the leftmost point is x=−2x=-2, not −1-1 (D false). Answer: B.
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