$C_1$ is the circle with centre at $O(0,0)$ and radius $4, C_2$ is a variable circle with centre at $(\alpha, \beta)$ and radius 5 . If the common chord of $C_1$ and $C_2$ has slope $\frac{3}{4}$ and of maximum length, then one of the possible values of $\alpha+\beta$ is

If the pair of tangents drawn to the circle $x^2+y^2=a^2$ from the point $(10,4)$ are perpendicular. then $a=$

If $x-4=0$ is the radical axis of two orthogonal cirlces out of which one is $x^2+y^2=36$, then the centre of the other circle is