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In Euclidean plane geometry, a quadrilateral is a polygon with four sides (or 'edges') and four vertices or corners. Sometimes, the term quadrangle is used, by analogy with triangle, and sometimes tetragon for consistency with pentagon (5-sided), hexagon (6-sided) and so on. The word quadrilateral is made of the words quad (meaning "four") and lateral (meaning "of sides").

Quadrilaterals are simple (not self-intersecting) or complex (self-intersecting), also called crossed. Simple quadrilaterals are either convex or concave.

The interior angles of a simple quadrilateral add up to 360 degrees of arc.

(Our solved example in mathguru.com also uses the below concept. This is our own explanation, it is not taken from Wikipedia.)

To construct a quadrilateral when 2 adjacent sides( x units, y units) and 3 angles(say 105, 105, 60) are known

Draw a line segment MO = x units

Make an angle of 105at O

With O as the centre and radius y units, we mark an arc on the ray OX

Make an angle of 105at R

Make an angle of 60at M

Mark the point of intersection at R