$PQRS$ is a parallelogram. If the two diagonals are equal,find the measure of $\angle PQR$. (in $^{\circ}$)

If $\Delta ABC \cong \Delta RPQ$ and the perimeter of $\Delta ABC$ is $18 \, cm$,then the perimeter of $\Delta PQR$ is $cm$.

In the figure,$ABC$ is a right triangle,right-angled at $B$,such that $\angle BCA = 2 \angle BAC$. Show that the hypotenuse $AC = 2 BC$.

For any convex quadrilateral $ABCD$,prove that $AB + BC + CD + DA < 2(AC + BD)$.

In $\Delta PQR$,$\angle Q = \angle R$ and $PQ = 6.5 \, cm$,then find $PR$. (in $, cm$)

There can be $\ldots \ldots \ldots$ one-one correspondences between two triangles.