Ta có
(a-x)y3-(a-y)x3+(x-y)a3
= ay3 +a2y2 - ax2y - a2xy -a2y2 -a3y + a2x2 +a3x - xy3 - axy2 +x3y + ax2y + axy2 + a2xy - ax3 - a2x2
= ay ( y2 +ay -x2 - ax ) -a2 ( y2 + ay -x2 -ax ) - xy ( y2 + ay - x2 -ax ) + ax ( y2 + ay -x2 -ax )
= ( y2 +ay -x2 -ax ) ( ay -a2 -xy + ax )
= ( y2 + xy +ay -xy -ax -x2 ) ( ( y -a )a - x( y-a ) )
= (y( y +x +a ) - x( y +x+a ))(a-x )(a-y)
= (y+x+a)(y-x)(a-x)(y-a)