Question

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

• A. $\sqrt{7}$
• B. $4$
• C. $3$
• D. $5$

Answer 1

The correct option is D $5$

Given ellipse can be written in standard form as $\frac{x^2}{16}+\frac{y^2}{9}=1$

So director circle of above ellipse is given by, $x^2+y^2=a^2+b^2=16+9$

$\Rightarrow x^2+y^2=25=5^2$

Hence radius of director circle is $5$