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JEE Advanced 2007 · Conic Sections

Tangents are drawn from the point (17,7)(17, 7) to the circle x2+y2=169x^{2} + y^{2} = 169. STATEMENT-1: The tangents are mutually perpendicular. because STATEMENT-2: The locus of the points from which mutually perpendicular tangents can be drawn to the given circle is x2+y2=338x^{2} + y^{2} = 338. (1) Statement-1 is True, statement-2 is True; Statement-2 is a correct explanation for Statement-1. (2) Statement-1 is True, Statement-2 is True; Statement-2 is NOT a correct explanation for Statement-1 (3) Statement-1 is True, Statement-2 is False (4) Statement-1 is False, Statement-2 is True.

Answer: A. Statement-1 is True, statement-2 is True; Statement-2 is a correct explanation for Statement-1.

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Written solution

x2+y2=169⇒centre O(0,0), r=13x^2 + y^2 = 169 \quad\Rightarrow\quad \text{centre } O(0,0),\ r = 13

P(17,7),OP2=172+72=338P(17, 7), \qquad OP^2 = 17^2 + 7^2 = 338

S-1: the two tangents from P are mutually perpendicular?\text{S-1: the two tangents from } P \text{ are mutually perpendicular?}

S-2: locus of such points is x2+y2=338?\text{S-2: locus of such points is } x^2 + y^2 = 338?

OP2=338>r2=169 ⇒ P outsideOP^2 = 338 > r^2 = 169 \ \Rightarrow\ P \text{ outside}

1. tangent length and contact points1.\ \text{tangent length and contact points}

2. angle between the two tangents2.\ \text{angle between the two tangents}

3. the locus (director circle)3.\ \text{the locus (director circle)}

PT=OP2−r2=338−169=169=13PT = \sqrt{OP^2 - r^2} = \sqrt{338 - 169} = \sqrt{169} = 13

tangent at (x1,y1): xx1+yy1=169\text{tangent at } (x_1, y_1):\ x x_1 + y y_1 = 169

P(17,7) lies on it ⇒ 17x1+7y1=169P(17,7) \text{ lies on it} \ \Rightarrow\ 17x_1 + 7y_1 = 169

⇒ T1(5,12),T2(12,−5)\Rightarrow\ T_1(5, 12), \quad T_2(12, -5)

OT ⊥ PT ⇒ ∠ OTP = 90^∘

sin⁡α=OTOP=13338=12\sin\alpha = \dfrac{OT}{OP} = \dfrac{13}{\sqrt{338}} = \dfrac{1}{\sqrt2}

⇒ α=45∘\Rightarrow\ \alpha = 45^\circ

angle between tangents=2α=90∘\text{angle between tangents} = 2\alpha = 90^\circ

⇒ Statement-1 is TRUE\Rightarrow\ \text{Statement-1 is TRUE}

director circle: OP2=2r2\text{director circle}: \ OP^2 = 2r^2

x2+y2=2r2=2(169)=338x^2 + y^2 = 2r^2 = 2(169) = 338

⇒ Statement-2 is TRUE\Rightarrow\ \text{Statement-2 is TRUE}

P(17,7): 172+72=338=2r2P(17,7):\ 17^2 + 7^2 = 338 = 2r^2

⇒ Statement-2 explains Statement-1\Rightarrow\ \text{Statement-2 explains Statement-1}

Both statements are TRUE; S-2 explains S-1\text{Both statements are TRUE; S-2 explains S-1}

Answer: Option (1)\boxed{\text{Answer: Option (1)}}

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