NCERT Solutions for Class 9 Maths Unit 8
Quadrilaterals Class 9
Unit 8 Quadrilaterals Exercise 8.1, 8.2 Solutions
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 origin of the word "quadrilateral" is the two Latin words quadri, a variant of four, and latus, meaning "side".
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 (and planar) quadrilateral ABCD add up to 360 degrees of arc, that is
A + B + C + D = 360 degree.
This is a special case of the n-gon interior angle sum formula (n - 2) × 180°. In a crossed quadrilateral, the four interior angles on either side of the crossing add up to 720°.
All convex quadrilaterals tile the plane by repeated rotation around the midpoints of their edges.