If the power of the point $(1, 6)$ with respect to the circle $x^2 + y^2 + 4x - 6y - a = 0$ is $-16$,then $a =$

The equation of the circle whose diameters have the end points $(a, 0)$ and $(0, b)$ is given by:

The equation of the circle with centre $(2, -3)$ and touching the $X$-axis is

The abscissae of the two points $A$ and $B$ are the roots of the equation $x^2+2ax-b^2=0$ and their ordinates are roots of the equation $y^2+2py-q^2=0$. Then the equation of the circle with $AB$ as diameter is given by

The sides of a square are $x = 2, x = 3, y = 1$ and $y = 2$. Find the equation of the circle drawn with the diagonal of the square as its diameter.

The centre and radius of the circle $2x^2 + 2y^2 - x = 0$ are