2 2 2A B C C B A 2 2 2B C A B AC2 2 22A 2B 2C 2C 2B 2A ...
Bài 2 :
2
2
2
a b c c b ab c a b ac2a 2b 2c 2c 2b 2a
0,5đ
b c a b a c2
2
2
2
2
2
a a b c c a b b c 0,25đ
2
22
2
22
2
22
0b c c a b b c a a a b c a b c 0