The distance between the centres of two circles of radii $3 cm$ and $8 cm$ is $13 cm$. The points of contact of a direct common tangent to the circles are $P$ and $Q$. The length of $PQ$ is _____.

The length of two chords $AB$ and $AC$ of a circle are $10 cm$ and $24 cm$. If the chord $AB$ is at a distance $12 cm$ from the centre, then the distance of the chord $AC$ from the centre is _____.

Two equal circles of radius $\text{'}r\text{'}$ intersect in such a way that the centre of each circle passes through the centre of the other. Then the length of the common chord is _____.