Let the circle $S: 36 x^{2}+36 y^{2}-108 x+120 y+C=0$ be such that it neither intersects nor touches the co-ordinate axes. If the point of intersection of the lines, $x-2 y=4$ and $2 x-y=5$ lies inside the circle $\mathrm{S}$, then :
Correct Option: , 4
$S: 36 x^{2}+36 y^{2}-108 x+120 y+C=0$
$\Rightarrow x^{2}+y^{2}-3 x+\frac{10}{3} y+\frac{C}{36}=0$
Centre $\equiv(-g,-f) \equiv\left(\frac{3}{2}, \frac{-10}{6}\right)$
radius $=r=\sqrt{\frac{9}{4}+\frac{100}{36}-\frac{C}{36}}$
Now,
$\Rightarrow \mathrm{r}<\frac{3}{2}$
$\Rightarrow \frac{9}{4}+\frac{100}{36}-\frac{\mathrm{C}}{36}<\frac{9}{4}$
$\Rightarrow \mathrm{C}>100$ .........(1)
Now point of intersection of $x-2 y=4$ and $2 x-y=5$ is $(2,-1)$, which lies inside the circle $S .$
$\therefore \mathrm{S}(2,-1)<0$
$\Rightarrow(2)^{2}+(-1)^{2}-3(2)+\frac{10}{3}(-1)+\frac{C}{36}<0$
$\Rightarrow 4+1-6-\frac{10}{3}+\frac{C}{36}<0$
$\mathrm{C}<156$ ..........(2)
From (1) \& (2)
$100<\mathrm{C}<156$ Ans.
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