Ch 9 extends mensuration to parallelograms, triangles, and circles. Students learn formulas for area and perimeter/circumference of these shapes and apply them to solve real-world problems.
Area of parallelogram = base × height (perpendicular height, not slant side). Area of triangle = ½ × base × height. Any side can be the base; the height is always the perpendicular distance from the base to the opposite vertex/side.
Circumference (perimeter) of a circle = 2πr = πd, where r = radius, d = diameter. Area of a circle = πr². Use π ≈ 22/7 or 3.14. A semicircle has area = πr²/2 and perimeter = πr + 2r.
Download: https://ncert.nic.in/textbook/pdf/gemh109.pdf | Complete book: https://ncert.nic.in/textbook/pdf/gemh1ps.zip
The area measures the space inside the shape. The height (perpendicular distance) determines how much space it covers, not the slant side. A parallelogram can be rearranged into a rectangle with the same base and height.
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