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JEE Advanced 2008 · Application of Derivatives

Consider the two curves C1:y2=4xC_1 : y^2 = 4x, C2:x2+y2−6x+1=0C_2 : x^2 + y^2 - 6x + 1 = 0. Then,

Answer: A. C1C_1 and C2C_2 touch each other only at one point.

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Written solution

Consider the two curves C₁ : y² = 4x, C₂ : x² + y² - 6x + 1 = 0. Then, C1:y2=4x(parabola)C_1: y^2 = 4x \quad (\text{parabola}) C2:x2+y2−6x+1=0(circle)C_2: x^2 + y^2 - 6x + 1 = 0 \quad (\text{circle}) Find: how C1 and C2 meet\text{Find: how } C_1 \text{ and } C_2 \text{ meet} C2:(x−3)2+y2=8C_2: (x-3)^2 + y^2 = 8 centre (3,0),radius =22\text{centre } (3,0), \quad \text{radius } = 2\sqrt{2} y2=4x:x2+4x−6x+1=0y^2 = 4x: \quad x^2 + 4x - 6x + 1 = 0 x2−2x+1=0⇒(x−1)2=0x^2 - 2x + 1 = 0 \Rightarrow (x-1)^2 = 0 x=1 (double root)x = 1 \text{ (double root)} y2=4⇒y=±2y^2 = 4 \Rightarrow y = \pm 2 Points of intersection: (1,2), (1,−2)\text{Points of intersection: } (1, 2), \ (1, -2) Parabola: y′=2y;Circle: y′=3−xy\text{Parabola: } y' = \frac{2}{y}; \quad \text{Circle: } y' = \frac{3-x}{y} At x=1: 2y=3−1y=2y ⇒common tangent\text{At } x=1: \ \frac{2}{y} = \frac{3-1}{y} = \frac{2}{y} \ \Rightarrow \text{common tangent} Slopes equal at (1,±2)⇒tangent at both\text{Slopes equal at } (1,\pm2) \Rightarrow \text{tangent at both} ∴Option (2): touch exactly at two points\therefore \text{Option } (2): \text{ touch exactly at two points}

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