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If $y=m x+c$ is tangent on the ellipse $\frac{x^2}{9}+\frac{y^2}{4}=1$, then the value of $c$ is
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The correct answer is:
$\pm \sqrt{9 m^2+4}$
Here, $a=3, b=2 . \therefore$ By formula, $c^2=b^2+a^2 m^2$
$\therefore c^2=4+9 m^2 ; \therefore c= \pm \sqrt{9 m^2+4} \text {. }$
$\therefore c^2=4+9 m^2 ; \therefore c= \pm \sqrt{9 m^2+4} \text {. }$
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