x^4-x^2y^2+16y^4=x^4+8x^2y^2+16y^4-9x^2y^2=(x²+4y²)-9x²y²=(x²+4y²-3xy)(x²+4y²+3xy)=(x+y)(x+3y)(x²-3xy+4y²)
原式=(x^2+4*y^2)^2-9*x^2y^2=(x^2+4*y^2)^2-(3xy)^2=(x^2+3xy+4*y^2)(x^2-3xy+4*y^2)
