{1-(1-a)^2}×{1-(1-a)^2}
={1-(1-a)^2}^2
={1-(1-2a+a^2)}^2
=(-a^2+2a)^2
=a^4-4a^3+4a^2
=a^2(a^2-4a+4)= a^2(4-4a+a^2)= a^2(2-a)^2