I am trying to find the centre of the equation mentioned in part c), but I can only really understand the "long way" of doing it (i.e., finding the intersection of the prependicular bisectors of $PT$ and $QT$). In the marking scheme, the quickest way of doing it is to find the midpoint of $QT$. This implies that $QT$ is a diameter but I simply cannot undretsand this since it seems to me that it certainly can be a chord. My question is whether we can, with the information given, detail why exactly $PQ$ is a diameter.
Thanks
(Note: By part "c" I mean "A second circle...")