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JEE Advanced 2017 Paper 1 · Mathematics · Conic Sections · hard

From the point T(-6,1), two tangents are drawn to the parabola y^2=4x, touching it at A and B.

One or more options correct

From the point T(−6,1)T(-6,1), two tangents are drawn to the parabola y2=4xy^{2}=4x, touching it at AA and BB. Which of the following is (are) correct?
  1. A. The chord of contact ABAB is 2x−y=122x-y=12
  2. B. The area of triangle TABTAB is 1252\dfrac{125}{2}
  3. C. The circumcircle of triangle TABTAB passes through the focus (1,0)(1,0)
  4. D. The tangents TATA and TBTB are perpendicular to each other
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Answer: A, B

The tangent at (t2,2t)(t^2,2t) is ty=x+t2ty=x+t^2. It passes through TT if t2−t−6=0t^2-t-6=0, so t=3,−2t=3,-2 and the contact points are A(9,6)A(9,6), B(4,−4)B(4,-4). (A) The chord of contact is TT-polar: y⋅1=2(x−6)y\cdot1=2(x-6), i.e. 2x−y=122x-y=12; it contains both AA and BB; correct. (B) Area =12∣−6(6+4)+9(−4−1)+4(1−6)∣=1252=\tfrac12\left|-6(6+4)+9(-4-1)+4(1-6)\right|=\tfrac{125}{2} (also 12∣t1−t2∣3\tfrac12|t_1-t_2|^3); correct. (C) The circumcircle of TABTAB is x2+y2−3x−7y−48=0x^2+y^2-3x-7y-48=0, which at (1,0)(1,0) gives −50≠0-50\ne0; the focus property belongs to the triangle formed by three tangents, not to the triangle with the contact points; incorrect. (D) The slopes are 13\tfrac13 and −12-\tfrac12, product −16≠−1-\tfrac16\ne-1 (the angle is 45∘45^\circ); incorrect. Answer: A and B.
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