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10-2. Parabola, Ellipse, Hyperbola
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With one focus of the hyperbola $\frac{{{x^2}}}{9}\,\, - \,\,\frac{{{y^2}}}{{16}}\,\, = \,\,1$ as the centre , a circle is drawn which is tangent to the hyperbola with no part of the circle being outside the hyperbola. The radius of the circle is
A
$less\ than$ $2$
B
$2$
C
$\frac{{11}}{3}$
D
$none$
Solution

$e^2 = 1 + \frac{{16}}{9}\,$ $= $ $\frac{{25}}{9}\,$
$\Rightarrow $ $e = \frac{5}{3}\,$
$\therefore$ focus $= (5, 0)$
Use reflection property to prove that circle cannot touch at two points.
It can only be tangent at the vertex $r = 5 – 3 = 2 $
Standard 11
Mathematics