The number of common tangents to the circles $x^2+y^2-x=0$ and $x^2+y^2+x=0$ is /are

The parametric equations of the circle $x^2+y^2-\mathrm{a} x-b y=0$ are

The equation of the tangent to the circle, given by $x=5 \cos \theta, y=5 \sin \theta$ at the point $\theta=\frac{\pi}{3}$ on it , is

Let PQ and RS be tangents at the extremities of the diameter PR of a circle of radius $r$. If PS and RQ intersect at a point X on the circumference of the circle, then 2 r equals