(1+y)的平方-2x的平方(1-y的平方)+x的四次方(1-y)的平方
=(1+y)^2-2*(1+y)*(x^2(1-y))+(x^2(1-y))^2
=(1+y-(x^2(1-y)))^2
=(1+y-x^2+x^2y)^2