2 2 2A B C C B A 2 2 2B C A B AC 2A22 2B22 2C22 2C 2B 2A ...
Bài 2 :
2
2
2
a b c c b ab c a b ac
2a
2
2
2b2
2
2c2
2
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