A, CHO A, B, C ≠ 0 2 2 2A B C A B CCMR

Câu 2:

a, Cho a, b, c ≠ 0

2

2

2

a

b

c

a

b

c

CMR:

b



c



a

  

b

c

a

b, Cho n



N, n > 1

CMR:

1

1

₂

1

₂

1

5



13

 

....

n n

(

1)



2

