原式=x^2y^2-x^4y^4-(1/81)
=x^2y^2(1-x^2y^2)-(1/81)
=(1+xy)(1-xy)x^y^2-(1/9)^2
={√[(1+xy)(1-xy)]xy+(1/9)}{√[(1+xy)(1-xy)]xy-(1/9)}