Solid geometry.
Task 14
Every walkthrough works the same way: first the construction — the trace of the cutting plane, the intersection points, the cross-section itself. Then coordinates, vectors — and the answer in one short formula.
cos α = |n₁ · n₂| / (|n₁| · |n₂|)
1
Constructing the cross-section
Step by step, right in 3D — you can rotate every line.
2
Proof
Part (a) — rigorous, but no extra words.
3
Calculation and answer
The coordinate method where it is genuinely shorter.