∫In△FAD, GH is the midline.

∴GH parallel, equal to 1/2AD.

∫BC parallel, equal to 1/2AD.

∴BC is parallel and equal to GH

The quadrilateral BCHG is a parallelogram.

In the right-angled trapezoidal ABEF,

∵G is the midpoint of FA, and BE is parallel, equal to 1/2FA.

∴BE parallel and equal to FG

So the quadrilateral BEFG is a parallelogram

So BG∨EF

And HC sigma BG.

∴HC∥EF

∴ chef * * *

CDEF*** aircraft.

20.( 1) As shown in the figure, ∫CG∨BF, ∴∠EBF (or other angles) are the angles formed by non-planar straight lines BE and CG.

In △BEF, ∠ EBF = 45, so the angle between BE and CG is 45.

(2) connect FH, BD, FO, HD EA, EA FB,

∴HD ∴ FB，

∴ Quadrilateral HFBD is a parallelogram,

∴HF∥BD