If two diameters of a circle intersect each other at right angles, then quadrilateral formed by joining their end points is a
square
Given that the two diameters AB and CD of a circle intersect each other at right angles.
AD, DB, BC and CA are joined forming a quadrilateral ABCD.
∵ The diagonals are equal and bisect each other at right angles.
∴ ACBD is a square.