(1) It is proved that there are AB ⊥ AD, AB ⊥ AC, AB ⊥ after A 1D⊥A 1B,A2C⊥A2B are folded into a triangular pyramid in the right-angled trapezoid A1AAA3D. A 1d = A3D。 ∴ A 1d = A3D = 10，A 1b = A2B = 4。 Over-d is DE⊥A2A3, vertical foot is e, and DE=8. In Rt△DEA3, e is obtained.

High school mathematics "solid geometry"

(1) It is proved that there are AB ⊥ AD, AB ⊥ AC, AB ⊥ after A 1D⊥A 1B,A2C⊥A2B are folded into a triangular pyramid in the right-angled trapezoid A1AAA3D. A 1d = A3D。 ∴ A 1d = A3D = 10，A 1b = A2B = 4。 Over-d is DE⊥A2A3, vertical foot is e, and DE=8. In Rt△DEA3, e is obtained.