... abcPcy ca!3(a 2 + b 2 )3(b 2 + c 2 )3(c 2 + a 2 )3 ,27 a 2 b 2 c 2 (a 2 + b 2 ) 2 (b 2 + c 2 ) 2 (c 2 + a 2 ) 2 (a 2 b 2 )(b 2 c 2 )(c 2 a 2 ) Xcy ca 2 b 2 + abcXcy ca!3Do ... cb 2 a 2 a 2 + b 2 , 4Xcy c(a b) 2 a 2 + b 2 9(a 2 b 2 )(b 2 c 2 )(c 2 a 2 )(a 2 + b 2 )(b 2 + c 2 )(c 2 + a 2 )Theo bất đẳng thức AM-GM,4Xcy c(a b) 2 a 2 + b 2 12 3s(a ... b 2 )(3a 2 + b 2 )Ycy ca 2 b 2 a 2 + b 2 , 27 Ycy c(a b) 2 (3a 2 2ab + 3b 2 )(a 2 + b 2 )(3a 2 + b 2 )Ycy c(a 2 b 2 )3(a 2 + b 2 )3, 27 Ycy c(3a 2 2ab + 3b 2 )(a 2 +...