Measure perpendicular height to compare triangle areas without relying on shape.
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First lesson in this subject: Complete the angle sum
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A triangle’s area is ½ × base × perpendicular height. Perpendicular means the height meets the base line at a right angle. A sloping side is not generally the height.
The first triangle fits a rectangle with base 8 and height 3. Its area is half the rectangle’s area: ½ × 8 × 3 = 12 square units. Moving the top vertex sideways keeps that area if the base and height stay fixed.
You are reading the opening. Members read every lesson and guide in full. A free account reads the first lesson in each subject in full.
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First lesson in this subject: Complete the angle sum
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Practice · Compare triangle areas, rectangles and composite figures. Change dimensions to test your reasoning.
Book · Explore measurements, transformations, coordinates and geometric relationships through diagrams and worked examples.
Previous lesson · Find a missing triangle angle from the two angles you know.
Next lesson · Count filled unit squares for area and exposed edges for perimeter.
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