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JEE Advanced 2008 · Conic Sections

Consider L1:2x+3y+p−3=0L_1 : 2x + 3y + p - 3 = 0 L2:2x+3y+p+3=0L_2 : 2x + 3y + p + 3 = 0 where pp is a real number, and C:x2+y2+6x−10y+30=0C : x^{2} + y^{2} + 6x - 10y + 30 = 0 STATEMENT - 1: If line L1L_1 is a chord of circle CC, then line L2L_2 is not always a diameter of circle CC and STATEMENT - 2: If line L1L_1 is a diameter of circle CC, then line L2L_2 is not a chord of circle CC

Answer: c

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Written solution

Consider L₁ : 2x + 3y + p - 3 = 0 L₂ : 2x + 3y + p + 3 = 0 where p is a real number, and C : x² + y² + 6x - 10y + 30 = 0 STATEMENT - 1: If line L₁ is a chord of circle C, then line L₂ is not always a diameter of circle C and STATEMENT - 2: If line L₁ is a diameter of circle C, then line L₂ is not a chord of circle C

C:x2+y2+6x−10y+30=0C : x^{2} + y^{2} + 6x - 10y + 30 = 0

(x+3)2+(y−5)2=4⇒centre (−3,5), r=2(x+3)^{2} + (y-5)^{2} = 4 \Rightarrow \text{centre } (-3, 5),\ r = 2

L1:2x+3y=3−p,L2:2x+3y=−3−p (parallel)L_1 : 2x + 3y = 3 - p,\qquad L_2 : 2x + 3y = -3 - p\ \text{(parallel)}

Decide: truth of Statement-1 and Statement-2,\text{Decide: truth of Statement-1 and Statement-2,}

and whether Statement-2 explains Statement-1\text{and whether Statement-2 explains Statement-1}

C:x2+y2+6x−10y+30=0C: x^{2}+y^{2}+6x-10y+30=0

(x+3)2+(y−5)2=4(x+3)^{2}+(y-5)^{2}=4

Centre O(−3,5),r=2\text{Centre } O(-3,5),\quad r=2

L1:2x+3y=3−p,L2:2x+3y=−3−pL_1: 2x+3y = 3-p,\qquad L_2: 2x+3y = -3-p

Distance between L1,L2=613≈1.66<2\text{Distance between } L_1, L_2 = \dfrac{6}{\sqrt{13}} \approx 1.66 < 2

1. Distances d1,d2 of L1,L2 from O\text{1. Distances } d_1, d_2 \text{ of } L_1, L_2 \text{ from } O

2. Chord   ⟺  d<2;Diameter  ⟺  d=0\text{2. Chord } \iff d<2;\quad \text{Diameter} \iff d=0

3. Test Statement-1, then Statement-2\text{3. Test Statement-1, then Statement-2}

d=∣ax0+by0+c∣a2+b2d = \dfrac{|a x_0 + b y_0 + c|}{\sqrt{a^{2}+b^{2}}}

d1=∣p+6∣13,d2=∣p+12∣13d_1 = \dfrac{|p+6|}{\sqrt{13}},\qquad d_2 = \dfrac{|p+12|}{\sqrt{13}}

Chord  ⟺  d<2\text{Chord} \iff d < 2

Diameter  ⟺  d=0\text{Diameter} \iff d = 0

p=−12: d2=0⇒L2 is a diameterp=-12:\ d_2 = 0 \Rightarrow L_2 \text{ is a diameter}

d1=613<2⇒L1 is a chordd_1 = \dfrac{6}{\sqrt{13}} < 2 \Rightarrow L_1 \text{ is a chord}

∴ Statement-1 is TRUE\therefore\ \text{Statement-1 is TRUE}

p=−6: d1=0⇒L1 is a diameterp=-6:\ d_1 = 0 \Rightarrow L_1 \text{ is a diameter}

d2=613<2⇒L2 is a chordd_2 = \dfrac{6}{\sqrt{13}} < 2 \Rightarrow L_2 \text{ is a chord}

∴ Statement-2 is FALSE\therefore\ \text{Statement-2 is FALSE}

Statement-1: TRUE,Statement-2: FALSE\text{Statement-1: TRUE},\qquad \text{Statement-2: FALSE}

Answer: S-1 True, S-2 False\text{Answer: S-1 True,\ S-2 False}

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