Question:
Find the equation of the circle concentric with the circle $x^{2}+y^{2}-4 x-6 y-3$ = 0 and which touches the y-axis.
Solution:
The given image of the circle is:
We know that the general equation of the circle is given by:
$x^{2}+y^{2}+2 g x+2 f y+c=0$
Also,
Radius r = $\sqrt{g^{2}+f^{2}-c}$
Now,
$r=\sqrt{(2)^{2}+(3)^{2}-(-3)}$
$r=\sqrt{4+9+3}$
$r=4$ units.
We need to the find the equation of the circle which is concentric to the qiven circle and touches y-axis.
The centre of the circle remains the same.
Now, y-axis will be tangent to the circle.
Point of contact will be $(0,3)$
Therefore, radius $=2$
Now,
Equation of the circle:
$(x-2)^{2}+(y-3)^{2}=(2)^{2}$
$x^{2}+4-4 x+y^{2}+9-6 y=4$
$x^{2}+y^{2}-4 x-6 y+9=0$