In the given figure, if $\mathrm{ABC}$ and $\mathrm{DEF}$ are two straight lines, then the value of $x$ is 

In the given figure, if $ABCD$ is a rhombus, then the value of $(x-y)$ is 

In a square $\mathrm{ABCD}$, if the diagonals bisect at $\mathrm{O}$. Then $∆\mathrm{AOB}$ is

For which of the following quadrilaterals, diagonals are perpendicular to each other?

A quadrilateral whose all sides are equal, opposite angles are equal and the diagonals bisect each other at right angles is a _____.

A quadrilateral has three acute angles. If each measure $80°$ and the measure of fourth angle is $x°$, then $x=$_____$°$.

A regular polygon is a polygon whose all sides are equal and all _____ are equal.

The sum of all exterior angles of a polygon is _____$°$.

The number of sides in a regular polygon having measure of an exterior angle as $72°$ is _____.