Quadrilaterals
A floor tiled with parallelograms and a window grill of rectangles both hide strict rules. Chapter 8, Quadrilaterals, of the NCERT Mathematics textbook for Class 9 begins with the angle-sum property and then studies the parallelogram in depth. You will prove that opposite sides and angles are equal and that diagonals bisect each other, meet the rectangle, rhombus and square as special cases, and use the mid-point theorem on the segment joining the mid-points of two sides. Riders and constructions build careful geometric reasoning.
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