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JEE Advanced 2024 · Conic Sections

Consider the ellipse x29+y24=1\frac{x^{2}}{9} + \frac{y^{2}}{4} = 1. Let S(p,q)S(p, q) be a point in the first quadrant such that p29+q24>1\frac{p^{2}}{9} + \frac{q^{2}}{4} > 1. Two tangents are drawn from SS to the ellipse, of which one meets the ellipse at one end point of the minor axis and the other meets the ellipse at a point TT in the fourth quadrant. Let RR be the vertex of the ellipse with positive xx-coordinate and OO be the center of the ellipse. If the area of the triangle ΔORT\Delta ORT is 32\frac{3}{2}, then which of the following options is correct?

Answer: A. q=2,p=33q = 2, p = 3\sqrt{3}

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Written solution

Consider the ellipse x²/9 + y²/4 = 1. Let S(p, q) be a point in the first quadrant such that p²/9 + q²/4 > 1. Two tangents are drawn from S to the ellipse, of which one meets the ellipse at one end point of the minor axis and the other meets the ellipse at a point T in the fourth quadrant. Let R be the vertex of the ellipse with positive x-coordinate and O be the center of the ellipse. If the area of the triangle Δ ORT is 3/2, then which of the follow

E: x29+y24=1,S(p,q), p,q>0E:\ \dfrac{x^{2}}{9}+\dfrac{y^{2}}{4}=1,\quad S(p,q),\ p,q>0

p29+q24>1 ⇒ S is outside E\dfrac{p^{2}}{9}+\dfrac{q^{2}}{4}>1 \ \Rightarrow\ S \text{ is outside } E

R=(3,0), O=(0,0), [triangle ORT]=3/2

Tangent 1: at a minor-axis end point\text{Tangent } 1: \text{ at a minor-axis end point}

Tangent 2: at T∈Q4\text{Tangent } 2: \text{ at } T \in \text{Q4}

To find: option giving (p,q)\text{To find: option giving } (p,q)

x29+y24=1⇒a=3, b=2\dfrac{x^{2}}{9}+\dfrac{y^{2}}{4}=1 \Rightarrow a=3,\ b=2

R=(3,0),minor-axis ends (0,±2)R=(3,0),\quad \text{minor-axis ends } (0,\pm 2)

1. tangent at a minor-axis end⇒q1.\ \text{tangent at a minor-axis end} \Rightarrow q

2. T=(3cos⁡θ,2sin⁡θ)⇒tangent at T2.\ T=(3\cos\theta,2\sin\theta) \Rightarrow \text{tangent at } T

  1. [triangle ORT] ⇒ θ ⇒ p

Tangent at (0,2): y⋅24=1⇒y=2\text{Tangent at } (0,2):\ \dfrac{y\cdot 2}{4}=1 \Rightarrow y=2

S(p,q)∈y=2⇒q=2S(p,q)\in y=2 \Rightarrow q=2

T=(3cos⁡θ, 2sin⁡θ)T=(3\cos\theta,\ 2\sin\theta)

Tangent at T: xcos⁡θ3+ysin⁡θ2=1\text{Tangent at } T:\ \dfrac{x\cos\theta}{3}+\dfrac{y\sin\theta}{2}=1

S(p,2)∈tangentT: pcos⁡θ3+sin⁡θ=1S(p,2)\in \text{tangent}_T:\ \dfrac{p\cos\theta}{3}+\sin\theta=1

[triangle ORT]=1/2|x_R y_T-x_T y_R|=1/2|3· 2sinθ|=3|sinθ|

3∣sin⁡θ∣=32⇒∣sin⁡θ∣=123|\sin\theta|=\dfrac{3}{2} \Rightarrow |\sin\theta|=\dfrac{1}{2}

T∈Q4: sin⁡θ=−12,cos⁡θ=32T\in \text{Q4}:\ \sin\theta=-\dfrac{1}{2},\quad \cos\theta=\dfrac{\sqrt{3}}{2}

p(3/2)3−12=1⇒p36=32\dfrac{p(\sqrt{3}/2)}{3}-\dfrac{1}{2}=1 \Rightarrow \dfrac{p\sqrt{3}}{6}=\dfrac{3}{2}

p3=9⇒p=33p\sqrt{3}=9 \Rightarrow p=3\sqrt{3}

q=2, p=33q=2,\ p=3\sqrt{3}

Option (1)\text{Option } (1)

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