(x²+y²-1)²-4x²y²
=[(x²+y²-1)+2xy]*[(x²+y²-1)-2xy]
=(x²+y²-1+2xy)*(x²+y²-1-2xy)
=[(x-y)^2 -1].[(x+y)^2 -1]
=(x-y-1)(x-y+1)(x+y-1)(x+y+1)。
原式
=(x²+y²-1+2xy)(x²+y²-1-2xy)
=[(x+y)²-1][(x-y)²-1]
=(x+y+1)(x+y-1)(x-y+1)(x-y-1)
(x^2+y^2-1)^2 -4x^2.y^2
=(x^2+y^2-1-2xy) .(x^2+y^2-1+2xy)
=[(x-y)^2 -1].[(x+y)^2 -1]
=(x-y-1)(x-y+1)(x+y-1)(x+y+1)
(x²+y²-1)²-4x²y²
=[(x²+y²-1)+2xy]*[(x²+y²-1)-2xy]
=(x²+y²-1+2xy)*(x²+y²-1-2xy)