with equality only if $ a = b $, i.e., $ racp+qp-q = racp-qp+q $, which implies $ (p+q)^2 = (p-q)^2 \Rightarrow pq = 0 $, impossible. So $ R > 0 $, but minimum not attained. But perhaps the hydrologist defines $ R $ only when flow is stable — yet no such condition - Groen Casting