Let the equation of a straight line passing through point $A(\alpha,\beta,\gamma)$ and having direction ratios $l,m,n$ be $\dfrac{x-\alpha}{l}=\dfrac{y-\beta}{m}=\dfrac{z-\gamma}{n}=r$ Suppose $P$ is any arbitrary point on this line with coordinates $(\alpha+lr,\beta+mr,\gamma+nr)$. Geometrically, $r$ is