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The radius of the director circle of the conic $9x^{2}+16y^{2}=144 $ is

\[\begin{array}{1 1}(1)\sqrt{7}&(2)4\\(3)3&(4)5\end{array}\]

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$9x^2+16y^2=144$
$\large\frac{9x^2}{144} +\frac{16y^2}{144} $$=1$
$\large\frac{x^2}{16} +\frac{y^2}{9} $$=1$
$a^2= 16$
$b^2=9$
Radius of the director circle is
$\qquad= \sqrt {a^2+b^2}$
$\qquad= \sqrt {16+9}$
$\qquad= \sqrt {25}$
$\qquad=5$
Hence 4 is the correct answer
answered May 16, 2014 by meena.p
 
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