Mathematics single ch Area of plain figures NOTES

Class 9 Mathematics

Chapter: Areas of Plane Figures


1. Plane Figures
A plane figure is a two-dimensional shape drawn on a flat surface. Examples include square,
rectangle, triangle, circle, parallelogram and trapezium.

Area is the measure of the surface enclosed by a plane figure.
Unit of area: square units such as cm², m².

2. Area of a Rectangle
A rectangle has opposite sides equal.
Formula: Area = Length × Breadth
Example: Length = 8 cm, Breadth = 5 cm
Area = 8 × 5 = 40 cm²

3. Area of a Square
All sides of a square are equal.
Formula: Area = Side²
Example: Side = 6 cm
Area = 6 × 6 = 36 cm²

4. Area of a Triangle
Triangle is half of a rectangle.
Formula: Area = ½ × Base × Height
Example: Base = 10 cm, Height = 6 cm
Area = ½ × 10 × 6 = 30 cm²

5. Area of a Parallelogram
Opposite sides are equal and parallel.
Formula: Area = Base × Height
Note: Height must be perpendicular to the base.

6. Area of a Trapezium
A trapezium has one pair of parallel sides.
Formula: Area = ½ × (Sum of parallel sides) × Height

Important Points
• Area is always written in square units.
• Height should be perpendicular.
• Draw diagrams neatly in exams.