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10-2. Parabola, Ellipse, Hyperbola
normal
If the variable line $y = kx + 2h$ is tangent to an ellipse $2x^2 + 3y^2 = 6$ , the locus of $P (h, k)$ is a conic $C$ whose eccentricity equals
A
$\frac{{\sqrt 5 }}{2}$
B
$\frac{{\sqrt 7 }}{3}$
C
$\frac{{\sqrt 7 }}{2}$
D
$\sqrt {\frac{7}{3}} $
Solution
By using condition of tangency,
we get $4 \mathrm{h}^{2}=3 \mathrm{k}^{2}+2$
$\therefore $ Locus of $\mathrm{P}(\mathrm{h}, \mathrm{k})$ is $4 \mathrm{x}^{2}-3 \mathrm{y}^{2}=2$ (which is hyperbola.)
Hence $e^{2}=1+\frac{4}{3} \Rightarrow e=\sqrt{\frac{7}{3}}$
Standard 11
Mathematics