... 2 2= + + + + + = + + Vy a2 + b2 + c2 ab +bc + caDấu = xảy tra khi a=b=c0,250,50,252b 1A=2 3 2 32 2 3 2 2 3 + + + + . A 2 3 2 322 4 2 3 2 4 2 3 + = + + + . =2 ... 2) với mọi m0,50,54 1,5 Ta có:( )( )( )abcaccbba 8 =++ + ( ) ( ) ( ) 0222 222 222 =++ ++ + abccacbabcabacabcbcba( ) ( ) ( )0 222 =++ abccbacabTa có: ( )02cab cba ,, ( )02cba ... )a b c d+ + a2 +2 ac + c2 + b2 + 2bd + d22 2 2 2( )( )a b c d+ + ac + bd (1)Nu ac + bd < 0 thỡ BT c c/mNu ac + bd 0 0,250,250,2525(1) ( a2 + b2 )(c2 + d2)...