Statement
If the diagonals of a parallelogram are perpendicular then it is a rhombus.

Given:
ABCD is a parallelogram in which AC and BD are perpendicular to each other.
To prove:
ABCD is a rhombus.
Proof:
Let AC and BD intersect at right angles at O.
AO=OC (diagonals bisect each other)
OD=OD (common side)
AD = DC
i.e., the adjacent sides are equal.
By definition, ABCD is a rhombus.
