In $\Delta ABC$,$P$ and $Q$ are the midpoints of $AB$ and $AC$ respectively. If $BC + PQ = 21 \text{ cm}$,then find the length of $BC$.

In trapezium $ABCD$,$AB \parallel CD$. If $\angle A = 80^{\circ}$ and $\angle B = 75^{\circ}$,then find $\angle D$. (in $^{\circ}$)

In quadrilateral $ABCD$,$\angle A = 100^{\circ}$,$\angle B = 80^{\circ}$,and $\angle C = 120^{\circ}$. Find $\angle D$. (in $^{\circ}$)

Two parallel lines are intersected by a transversal. Show that the quadrilateral formed by the bisectors of interior angles is a rectangle.

Show that the quadrilateral formed by joining the mid-points of the consecutive sides of a square is also a square.

$PQ$ and $RS$ are two equal and parallel line segments. Any point $M$ not lying on $PQ$ or $RS$ is joined to $Q$ and $S$. Lines are drawn through $P$ parallel to $QM$ and through $R$ parallel to $SM$,meeting at $N$. Prove that line segments $MN$ and $PQ$ are equal and parallel to each other.