(3) (x+y+z)(-x+y+z)(x-y+z)(x+y-z)
={x+(y+z)}{−x+(y+z)}>{x-(y-z)}{x+(y-z)})(
={(y+z)²=x2}{x²-(y-z)²}
={-x²+(y+2)²}{x²-(y-z)}
x²+{(y+z)²+(y=z)²}x²=(y+z)²(y-z)²) (1-2
=-x+2(y²+z2) x² - (y²-z²)²
=-x1-y¹-z+2x²y²+2y²z²+2z²x²
(4) (x+y+1)(x²-xy+y²-x-y+1)
={x+(y+1)}{x²-(y+1)x+(y²−y+1)}
=x³+{(y+1)−(y+1)}x²+{(y²−y+1)−(y+1)²}x
+(y+1)(y²-y+1)
=x³+(-3y)x+y³+1
=x³+y³-3xy+1
