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Because the lines intersecting the three planes are not parallel, it can be seen that the extension lines of the three intersecting lines intersect, and the three lines must intersect if they are not parallel, because the intersection point is on the same plane as the extension lines of planes A and B, and on the same plane as the extension lines of planes B and C, and it must be the same point. plane

In triangle ane and triangle DGN, E and N are on the same plane, so the intersecting ANE angles DNG are equal. Because AB is parallel to DC, the angle 1 and the angle 2 are equal.

It can be concluded that the remaining angle 4 of two triangles is equal to angle 3, and AE is parallel to DG? So .. must be a straight line. Similarly, angle EBF is equal to angle GCF, angle BFE is equal to angle CFG, and it is concluded that angle BEF is equal to angle DGF, and EG and GF are in a straight line. So EGFN is a straight line.