JEE Advanced · Mathematics · 14. Ellipse
Consider two straight lines, each of which is tangent to both the circle and the parabola . Let these lines intersect at the point . Consider the ellipse whose center is at the origin and whose semi-major axis is . If the length of the minor axis of this ellipse is , then the which of the following statement(s) is (are) TRUE?
- A For the ellipse, the eccentricity is and the length of the latus rectum is 1
- B For the ellipse, the eccentricity is and the length of the latus rectum is
- C The area of the region bounded by the ellipse between the lines and is
- D The area of the region bounded by the ellipse between the lines and is
Answer & Solution
Correct Answer
(C) The area of the region bounded by the ellipse between the lines and is
Step-by-step Solution
Detailed explanation
Let equation of common tangent is \(\mathrm{y}=\mathrm{m} \mathrm{x}+\frac{1}{\mathrm{~m}}\)
\(\therefore\left|\frac{0+0+\frac{1}{\mathrm{~m}}}{\sqrt{1+\mathrm{m}^{2}}}\right|=\frac{1}{\sqrt{2}} \Rightarrow \mathrm{m}^{4}+\mathrm{m}^{2}-2=0 \Rightarrow \mathrm{m}=\pm 1\) Equation of common tangents are \(\mathrm{y}=\mathrm{x}+1\) and \(\mathrm{y}=-\mathrm{x}+1\)
point \(Q\) is \((-1,0)\)
\(\therefore\) Equation of ellipse is \(\frac{x^{2}}{1}+\frac{y^{2}}{1 / 2}=1\)
(A) \(\mathrm{e}=\sqrt{1-\frac{1}{2}}=\frac{1}{\sqrt{2}}\) and \(\mathrm{LR}=\frac{2 \mathrm{~b}^{2}}{\mathrm{a}}=1\)
(C) Area \(2 \cdot \int_{1 / \sqrt{2}}^{1} \frac{1}{\sqrt{2}} \cdot \sqrt{1-\mathrm{x}^{2}} \mathrm{dx}=\sqrt{2}\left[\frac{\mathrm{x}}{2} \sqrt{1-\mathrm{x}^{2}}+\frac{1}{2} \sin ^{-1} \mathrm{x}\right]_{1 / \sqrt{2}}^{1}\) \(=\sqrt{2}\left[\frac{\pi}{4}-\left(\frac{1}{4}+\frac{\pi}{8}\right)\right]=\sqrt{2}\left(\frac{\pi}{8}-\frac{1}{4}\right]=\frac{\pi-2}{4 \sqrt{2}}\)
Correct answers are (A) and (C).
\(\therefore\left|\frac{0+0+\frac{1}{\mathrm{~m}}}{\sqrt{1+\mathrm{m}^{2}}}\right|=\frac{1}{\sqrt{2}} \Rightarrow \mathrm{m}^{4}+\mathrm{m}^{2}-2=0 \Rightarrow \mathrm{m}=\pm 1\) Equation of common tangents are \(\mathrm{y}=\mathrm{x}+1\) and \(\mathrm{y}=-\mathrm{x}+1\)
point \(Q\) is \((-1,0)\)
\(\therefore\) Equation of ellipse is \(\frac{x^{2}}{1}+\frac{y^{2}}{1 / 2}=1\)
(A) \(\mathrm{e}=\sqrt{1-\frac{1}{2}}=\frac{1}{\sqrt{2}}\) and \(\mathrm{LR}=\frac{2 \mathrm{~b}^{2}}{\mathrm{a}}=1\)
(C) Area \(2 \cdot \int_{1 / \sqrt{2}}^{1} \frac{1}{\sqrt{2}} \cdot \sqrt{1-\mathrm{x}^{2}} \mathrm{dx}=\sqrt{2}\left[\frac{\mathrm{x}}{2} \sqrt{1-\mathrm{x}^{2}}+\frac{1}{2} \sin ^{-1} \mathrm{x}\right]_{1 / \sqrt{2}}^{1}\) \(=\sqrt{2}\left[\frac{\pi}{4}-\left(\frac{1}{4}+\frac{\pi}{8}\right)\right]=\sqrt{2}\left(\frac{\pi}{8}-\frac{1}{4}\right]=\frac{\pi-2}{4 \sqrt{2}}\)
Correct answers are (A) and (C).

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