How to prove this problem with the theorem of three perpendicular lines in the proof of solid geometry in senior high school mathematics?

Prove the vertical plane A 1B 1C of the train of thought, as long as two intersecting line segments in the vertical plane A 1B 1C of MN are proved.

First vertical

Connecting MC, A 1M, it is easy to get MC=MA 1, MN is perpendicular to CA 1, and a vertical line comes out.

Second vertical

Taking the midpoint N 1 of CB 1 and connecting BN 1 and NN 1, we can get that NMBN 1 is a parallelogram (NN 1 is parallel and equal to BM), and MN is parallel to BN 1.

At this point, you need sufficient conditions for the three vertical theorems.

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