The front and back of the cube are filled with the same cube, and the symmetry point of point P relative to the plane CC 1D 1D is P 1 (P 1 is the lower edge of the cube filled behind, and P 1 is the midpoint).

Then P 1, m, n three-point * * * line,

The point where the point Q is symmetrical with respect to the surface ABB 1A 1 is Q 1, (Q 1 is located at the upper left edge of the previously added cube, and Q 1 is the midpoint).

So the four-point * * line of P 1, m, n, Q 1, and then P 1Q! seek