Sides, Angles and diagonals of a quadrilateral; Different types of quadrilaterals: Trapezium, parallelogram, rectangle, rhombus and square.

• Sum of the angles of a quadrilateral is 360º,

• A diagonal of a parallelogram divides it into two congruent triangles,

• In a parallelogram

(i) opposite angles are equal

(ii) opposite sides are equal

(iii) diagonals bisect each other.

• A quadrilateral is a parallelogram, if

(i) its opposite angles are equal

(ii) its opposite sides are equal

(iii) its diagonals bisect each other

(iv) a pair of opposite sides is equal and parallel.

• Diagonals of a rectangle bisect each other and are equal and vice-versa

• Diagonals of a rhombus bisect each other at right angles and vice-versa

• Diagonals of a square bisect each other at right angles and are equal and vice-versa

• The line-segment joining the mid-points of any two sides of a triangle is parallel to the third side and is half of it

A line drawn through the mid-point of a side of a triangle parallel to another side bisects the third side,

• The quadrilateral formed by joining the mid-points of the sides of a quadrilateral, taken in order, is a parallelogram.