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