Вправи: 422

422. Скоротіть дріб

x2- y2- z2+2yzy2- x2- z2-2xz = x2-(y2 -2yz+z2)y2-(x2 +2xz+z2) = x2-(y- z)2y2- (x+ z)2 = (x- y+ z)(x+ y- z)(y- x- z)(y+ x+ z) = -(y- x- z)(x+ y- z)(y- x- z)(y+ x+ z) = z- x- yz+ x+ y.