Đáp án:
$6.$
$A = x² - 2xy - 4z² + y² $
$A = x² - 2xy + y² - 4z² $
$A = (x - y)² - (2z)² $
$A = (x - y - 2z)(x - y + 2z) $
$Với $ $ x = 6 ; y = - 4 ; z = 45 $
$⇒ A = [6 - (-4) - 2.45].[6 - (-4) + 2.45] $
$⇔ A = -80 . 100 $
$⇔ A = - 8000 $
$7.$
$xy(x + y) + yz(y + z) + zx(z + x) + 2xyz $
$= xy(x + y) + yz(y + z) + xyz + xz(x + z) + xyz$
$= xy(x + y) + yz(y + z + x) + xz(x + z + y)$
$= xy(x + y) + z(x + y + z)(x + y)$
$= xy(x + y) + (xz + zy + z²)(x + y) $
$= (x + y)(xy + xz + zy + z²)$
$= (x + y)[x(y + z) + z(y + z)]$
$= (x + y)(y + z)(x + z)$
