EG= 1/2CD= 1

∴ ef = (2 2+1) under radical number = 5.

(2) Using vector knowledge to solve.

AC 1 = a b+ bb 1+b 1c 1

(ac 1)^2=(ab+bb 1+b 1c 1)^2

(AC 1)^2=(a+b+a)^2

=(2a+b)^2

=4a^2+b^2+4ab

∴AC 1= under the radical sign (4a 2+b 2+4ab)

③ Take the midpoint e of BC.

Yizhi PE⊥BC

You: face PBC⊥ face ABCD

∴PE⊥BD

∴BD⊥ Face PAE

∴PA⊥BD

Take the midpoint f of PA.

It's easy to know BF⊥PA.

∵PA⊥BD again.

∴PA⊥ face BFD