Unfold the permitted faces, check that the straight segment stays inside them, then use the perpendicular offsets.
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First lesson in this subject: Complete the angle sum
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Keep the route on the two permitted faces. Unfold them into one plane, then join A and B with a straight segment. If it stays within those faces, its length is the square root of the sum of the two squared perpendicular offsets.
A and B are opposite corners. Stay on these faces only: front and top.
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First lesson in this subject: Complete the angle sum
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Practice · Unfold a box to compare exact distances across its faces, then fold each route back onto the solid.
Book · Explore measurements, transformations, coordinates and geometric relationships through diagrams and worked examples.
Previous lesson · Check that the drawing is connected, then count the dots with an odd number of lines.
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