( x + y + z )2 + ( x + y - z )2 - 4z2
= [ ( x + y ) + z ]2 + [ ( x + y ) - z ]2 - 4z2 (1)
Đặt \(\hept{\begin{cases}x+y=a\\z=b\end{cases}}\)
(1) <=> ( a + b )2 + ( a - b )2 - 4b2
= a2 + 2ab + b2 + a2 - 2ab + b2 - 4b2
= 2a2 - 2b2
= 2( a2 - b2 )
= 2( a - b )( a + b )
= 2( x + y - z )( x + y + z )