EASA / CAR Part-66 · Module 1 · 1.3 Geometry · Cat B1/B2
Objective: the theorem you know for a flat triangle works just as well through a solid. Find the diagonal across the floor first, then use that as one side of a second triangle to reach the far top corner.
Do it in two steps, not one. First find the diagonal across the floor using a and b. That diagonal then becomes the base of a second right triangle standing upright, with the height c as the other side, and its hypotenuse is the space diagonal.
The two steps collapse into one formula. Because the floor diagonal squared is already a² + b², substituting it into the second triangle gives √(a² + b² + c²) directly. Knowing where it comes from means you can rebuild it if you forget it.
This is how cable runs and bracing lengths are worked out. A wire running corner to corner through a bay, or a strut across a box structure, is exactly this calculation, which is why it appears in Module 1 and again in structures.