$ABCD$ is a parallelogram. If its diagonals are equal,then find the value of $\angle ABC$. (in $^{\circ}$)

If bisectors of $\angle A$ and $\angle B$ of a quadrilateral $ABCD$ intersect each other at $P$,of $\angle B$ and $\angle C$ at $Q$,of $\angle C$ and $\angle D$ at $R$,and of $\angle D$ and $\angle A$ at $S$,then $PQRS$ is a

If angles $A, B, C$ and $D$ of the quadrilateral $ABCD$,taken in order,are in the ratio $3:7:6:4$,then $ABCD$ is a

Diagonals of a parallelogram $ABCD$ intersect at $O.$ If $\angle BOC = 90^{\circ}$ and $\angle BDC = 50^{\circ},$ then $\angle OAB$ is (in $^{\circ}$)

The quadrilateral formed by joining the mid-points of the sides of a quadrilateral $PQRS,$ taken in order,is a rhombus,if

If $APB$ and $CQD$ are two parallel lines,then the bisectors of the angles $APQ, BPQ, CQP$ and $PQD$ form