The points P (a, b), Q (c, d), R (a, d) and S ( c, b), where a, b, c, dare distinct real numbers, are
collinear
vertices of a square
vertices of a rhombus
concyclic
We have,
Slope of PS=0=Slope of QR
and,
Slope of PR=∞=Slope of QS
⇒ PS and QR are parallel to x-axis and PR and QS are parallel to y-axis.
⇒ PQRS is a rectangle.
Also, PQ=RS.
Hence, PQRS is a square.