`A = ( 1 )/( xy + x + 1 ) + 1/( yz + y + 1 ) + 1/( zx + z + 1 )`
`A = 1/( xy + x + 1 ) + x/( xyz + xy + x ) + ( xy )/( x . xyz + xyz + xy )`
`A = 1/( xy + x + 1 ) + x/( 1 + xy + x ) + (xy)/( x + 1 + xy )`
`A = ( 1 + x + xy )/( xy + x + 1 )`
`A = 1`
Vậy `A = 1`