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JEE Advanced 2019 Paper 2 · Mathematics · Conic Sections · medium

The normal to the parabola y^2=4x at P(4,4) meets the parabola again at Q. Let F be the focus of the parabola.

One or more options correct

The normal to the parabola y2=4xy^2=4x at P(4,4)P(4,4) meets the parabola again at QQ. Let FF be the focus of the parabola. Which of the following statements is (are) correct?
  1. A. Q=(9,6)Q=(9,6)
  2. B. PQ=10PQ=10
  3. C. The area of triangle FPQFPQ is 252\dfrac{25}{2}.
  4. D. The area of the region bounded by the chord PQPQ and the parabola is 1253\dfrac{125}{3}.
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Answer: D

P(4,4)P(4,4) corresponds to the parameter t1=2t_1=2 on (t2,2t)(t^2,2t). The normal at t1t_1 meets the parabola again at t2=−t1−2t1=−3t_2=-t_1-\frac2{t_1}=-3, so Q=(9,−6)Q=(9,-6); option (A) with (9,6)(9,6) is wrong. PQ=52+102=55≠10PQ=\sqrt{5^2+10^2}=5\sqrt5\neq10, so (B) is wrong. With F=(1,0)F=(1,0) the area of FPQFPQ is 12∣(4−1)(−6−0)−(9−1)(4−0)∣=12∣−18−32∣=25\frac12\left|(4-1)(-6-0)-(9-1)(4-0)\right|=\frac12|-18-32|=25, not 252\frac{25}2, so (C) is wrong. For (D) integrate in yy from −6-6 to 44: the chord is x=4−12(y−4)x=4-\frac12(y-4) and the parabola is x=y24x=\frac{y^2}4; since the difference is a quadratic with leading coefficient 14\frac14, the enclosed area is (Δy)36⋅4=100024=1253\frac{(\Delta y)^3}{6\cdot4}=\frac{1000}{24}=\frac{125}3: correct. Answer: D only.
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