IGCSE Maths / Pythagoras theorem
The space diagonal of a cuboid revision notes
Edexcel International GCSE Maths (4MA1), chapter 15: Pythagoras theorem, specification point 4.8d. Part of the full course.
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The space diagonal is the longest straight line inside a cuboid, joining opposite corners. You can find it using Pythagoras twice, or use the cuboid shortcut, which comes from those two right-angled triangles.