Include the larger triangles and rectangles formed by joining small regions, and avoid counting an outline twice.
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First lesson in this subject: Complete the angle sum
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The line down the middle divides this drawing into two small triangular regions. The outside boundary is also a complete triangle.
That gives three triangles: the left half, the right half and the whole. An interior line does not erase a larger outline.
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You are reading the opening. Your free account reads the first lesson in each subject in full. Members read every lesson and guide in full.
First lesson in this subject: Complete the angle sum
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Practice · Count every triangle or rectangle in a line diagram, including larger figures made from smaller regions.
Book · Explore measurements, transformations, coordinates and geometric relationships through diagrams and worked examples.
Previous lesson · Find the relationship that survives changes of shape, size and direction, then check a new member.
Next lesson · Check that the drawing is connected, then count the dots with an odd number of lines.
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