A8 +B8 +C8 +D8 ≥4A2B2C2D2

3) - DE THI HSG TOAN 6 7 8 9
DE THI HSG TOAN 6 7 8 9

3) Chứng minh rằng:

a

8

+

b

8

+

c

8

+

d

8

≥

4

a

2

b

2

c

2

d

2