A 1N is parallel to MB NC, so it can be judged that c1m.
2. The first question shows that the problem of A 1C being parallel to C 1B is transformed into finding the included angle between C 1B and c1n..
Because AB is perpendicular to CN and C 1C, AB is perpendicular to c1n. Find the included angle in the right triangle C 1NB (is there a condition missing? NB=？ C 1N=？ )
If the topic is right-angled triangular prism, the second and third questions will be easy.