Class 9 · Chapter 8

Quadrilaterals

Properties of parallelograms, rectangles, rhombus, squares, trapezium; midpoint theorem.

Watch this chapter

Read the full chapter

Tip: open the NCERT PDF for the original chapter text alongside the notes here.

Detailed theory

Sum of the four interior angles of any quadrilateral is

360^\circ

. Proof: diagonal divides the quadrilateral into two triangles, each with angle sum

180^\circ

Key concepts & formulas

Sum of interior angles of a quadrilateral

= 360^\circ

Opposite sides of a parallelogram are equal.

Diagonals of a parallelogram bisect each other.

Diagonals of a rhombus bisect each other at

90^\circ

Midpoint theorem: line joining midpoints of two sides of a triangle is parallel to and half of the third side.

Every rectangle is a parallelogram; every square is both a rectangle and a rhombus.