arthham

JEE Advanced 2018 · Conic Sections

Consider two straight lines, each of which is tangent to both the circle x2+y2=12x^{2} + y^{2} = \frac{1}{2} and the parabola y2=4xy^{2} = 4x. Let these lines intersect at the point QQ. Consider the ellipse whose center is at the origin O(0,0)O(0,0) and whose semi-major axis is OQOQ. If the length of the minor axis of this ellipse is 2\sqrt{2}, then which of the following statement(s) is(are) TRUE?

Answers: A. For the ellipse, the eccentricity is 12\frac{1}{\sqrt{2}} and the length of the latus rectum is 11 · C. The area of the region bounded by the ellipse between the lines x=12x = \frac{1}{\sqrt{2}} and x=1x = 1 is 142(π−2)\frac{1}{4\sqrt{2}}(\pi - 2)

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Two lines tangent to both x2+y2=12 and y2=4x\text{Two lines tangent to both } x^2 + y^2 = \frac{1}{2} \text{ and } y^2 = 4x They intersect at Q\text{They intersect at } Q Ellipse centre O(0,0), semi-major axis =OQ\text{Ellipse centre } O(0,0), \text{ semi-major axis } = OQ Minor axis of the ellipse=2\text{Minor axis of the ellipse} = \sqrt{2} Which statements are true?\text{Which statements are true?} x2+y2=12: radius 12x^2 + y^2 = \frac{1}{2}: \ \text{radius } \frac{1}{\sqrt{2}} y2=4x:a=1y^2 = 4x: \quad a = 1 Tangent to y2=4x:y=mx+1m\text{Tangent to } y^2 = 4x: \quad y = mx + \frac{1}{m} d(O,line)=1∣m∣m2+1=12d(O, \text{line}) = \frac{1}{|m|\sqrt{m^2+1}} = \frac{1}{\sqrt{2}} m2(m2+1)=2 ⇒ m4+m2−2=0m^2(m^2+1) = 2 \ \Rightarrow \ m^4 + m^2 - 2 = 0 (m2+2)(m2−1)=0 ⇒ m2=1(m^2+2)(m^2-1) = 0 \ \Rightarrow \ m^2 = 1 m=±1m = \pm 1 L1:y=x+1,L2:y=−x−1L_1: y = x + 1, \qquad L_2: y = -x - 1 Q=L1∩L2=(−1,0)Q = L_1 \cap L_2 = (-1, 0) OQ=(−1)2+02=1OQ = \sqrt{(-1)^2 + 0^2} = 1 semi-major axis a=OQ=1\text{semi-major axis } a = OQ = 1 minor axis=2⇒b=22=12\text{minor axis} = \sqrt{2} \Rightarrow b = \frac{\sqrt{2}}{2} = \frac{1}{\sqrt{2}} x212+y2b2=1\frac{x^2}{1^2} + \frac{y^2}{b^2} = 1 e=1−b2a2=1−12=12e = \sqrt{1 - \frac{b^2}{a^2}} = \sqrt{1 - \frac{1}{2}} = \frac{1}{\sqrt{2}} Latus rectum=2b2a=2⋅121=1\text{Latus rectum} = \frac{2b^2}{a} = \frac{2 \cdot \frac{1}{2}}{1} = 1 x2+2y2=1⇒y=±121−x2x^2 + 2y^2 = 1 \Rightarrow y = \pm \frac{1}{\sqrt{2}}\sqrt{1-x^2} A=∫1/212⋅121−x2 dx=2∫1/211−x2 dxA = \int_{1/\sqrt{2}}^{1} 2\cdot\frac{1}{\sqrt{2}}\sqrt{1-x^2}\,dx = \sqrt{2}\int_{1/\sqrt{2}}^{1}\sqrt{1-x^2}\,dx ∫1−x2 dx=x1−x2+sin⁡−1x2\int \sqrt{1-x^2}\,dx = \frac{x\sqrt{1-x^2} + \sin^{-1}x}{2} ∫1/211−x2 dx=π4−(14+π8)=π8−14\int_{1/\sqrt{2}}^{1}\sqrt{1-x^2}\,dx = \frac{\pi}{4} - \left(\frac{1}{4} + \frac{\pi}{8}\right) = \frac{\pi}{8} - \frac{1}{4} A=2(π8−14)=142(π−2)A = \sqrt{2}\left(\frac{\pi}{8} - \frac{1}{4}\right) = \frac{1}{4\sqrt{2}}(\pi - 2) ∴Statements (1) and (3) are TRUE\therefore \text{Statements (1) and (3) are TRUE}

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