A:especially
B:suddenly
C:usually
D:really
-Your room is in a mess, David.
-I'm sorry. I'll __________ right now.
(Ⅰ)证明:a,b,c成等比数列;
(Ⅱ)若角B的平分线BD交AC于点D,且b=6,S△BAD=2S△BCD,求BD.