Check that the drawing is connected, then count the dots with an odd number of lines.
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First lesson in this subject: Complete the angle sum
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Count the lines meeting each dot. An arrival usually pairs with a departure. For one connected drawing, no odd counts means a closed trace; exactly two odd counts means those dots are the ends. Any other odd count makes a complete trace impossible.
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First lesson in this subject: Complete the angle sum
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Practice · Use every line once. Reason about connections, endpoints and the effect of adding one line.
Book · Explore measurements, transformations, coordinates and geometric relationships through diagrams and worked examples.
Previous lesson · Include the larger triangles and rectangles formed by joining small regions, and avoid counting an outline twice.
Next lesson · Unfold the permitted faces, check that the straight segment stays inside them, then use the perpendicular offsets.
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