Let the slope of a diameter $A C$ of a circle of radius 25 units be $\frac{3}{4}$. If $(3,2)$ is the centre of the circle, $A=\left(x_1, y_1\right)$ and $C=\left(x_2, y_2\right)$, then $\frac{x_1 x_2}{y_1 y_2}=$

A circle passes through the points $(1,2)$, $(3,4)$. If its centre lines on the line $x-y+3=0$, then its radius is equal to

A line drawn through the point $A(5,7)$ cut the circle $x^2+y^2-36=0$ at the points $P$ and $Q$. Then, $A P \cdot A Q=$

Let $P$ be any point on the circle $x^2+y^2-2 x-1=0$ and $C$ be its centre. Let $A B$ be the chord of contact of $P$ with respect to the circle $x^2+y^2-2 x=0$. Then, the locus of the circumcentre of the $\triangle C A B$ is