... ⇔=−=−=−000cbcaba(0 ,25 ) ⇔ a = b = c (0,5)Bài 3: (2 )a) (1đ) A = 2a 2 b 2 + 2b 2 c 2 + 2a 2 c 2 - a 4 - b 4 - c 4 = 4a 2 b 2 - ( a 4 + 2a 2 b 2 + b 4 ) + (2b 2 c 2 + 2a 2 c 2 ) - c 4 (0,5) = (2ab) 2 - ... = (2ab) 2 - [(a 2 +b 2 ) 2 -2c 2 (a 2 +b 2 )+c 4 ] = (2ab) 2 -[(a 2 +b 2 ) - c 2 ] 2 (0 ,25 ) = (2ab + a 2 + b 2 - c 2 )(2ab - a 2 - b 2 +c 2 ) = (a+b+c)(a+b-c)(c+a-b)(c-a+b) (0 ,25 )b) (1đ) ... A = a 2 (2a - 3) + b 2 (-3 + 2b) = 2( a3+b3)-3(a 2 +b 2 ) (0 ,25 ) = 2( a+b)(a 2 –ab + b 2 ) -3(a 2 +b 2 ) (0 ,25 ) = 2( a 2 –ab + b 2 ) -3(a 2 +b 2 ) (vì a+b=1) (0 ,25 ) = -2ab-a 2 -b 2 =...