Let $P$ be the point of intersection of the lines $L_1 \equiv x-y-7=0$ and $L_2 \equiv x+y-5=0 . A\left(x_1, y_1\right)$ and $B\left(x_2, y_2\right)$ are points on the lines $L_1=0$ and $L_2=0$ respectively such that $P A=3 \sqrt{2}$, $P B=\sqrt{2}, x_1, y_1 \geq 0, x_2, y_2 \geq 0$, then the angle made by the line segment $A B$ at the origin is

If the poles of the line $x-y=0$ with respect to the circles $x^2+y^2-2 g_i x+c_i^2=0(i=1,2,3)$ are ( $\alpha_i, \beta_i$ ), then $\sum_{i=1}^3 \frac{\alpha_i+\beta_i}{g_i}=$

If the circles $x^2+y^2-4 x+6 y+13-a^2=0$ and $x^2+y^2-10 x-2 y+17=0$ intersect in two distinct points, then ' $a$ ' is

If a circle of radius $r$ touches the positive coordinate axes and also the circle $x^2+y^2-12 x-10 y+52=0$ externally, then the distance between the centres of the two circles is