2 1 1 1 9   (1) 1 AB1 BC1 CA 2AB BC CA0,5    TA CÓ

4.2 1 1 1 9   (1) 1 ab1 bc1 ca 2ab bc ca0,5    Ta có:

 

^{1 3}   1 1 1 22 2ab ab ab ab        

2

2

2

2

2

2

2

2

2

1 2 2 2 2 2ab a b c ab  a b c ab a b cTheo Bunhiacopxki thì:

 

²

   

2

2

a b a b ab4     

2

2

2

2

2

2

2

2

2

2

a c b c a b c a b c    

2

2

2

2

2 1 1ab a b ab a b               

2

2

2

2

2

2

2

2

2

2

2

a b c a c b c ab a c b c2 2 1 2Tương tự:





bc

b

c

1



















1

2

bc

b

a

c

a



 

^{1 3}   (đfcm)

ca

c

a

ca

c

b

a

b

Dấu “=” xảy ra khi

¹

a

  

b

c

3