(a^2-1)(a^2+1)(a-1）-（a^2-1)^2
=(a^2-1)[(a^2+1)(a-1）-(a^2-1)]
=(a^2-1)(a^3-a^2+a-1-a^2+1)
=(a+1)(a-1)(a^3-2a^2+a)
=a(a+1)(a-1)(a^2-2a+1)
=a(a+1)(a-1)^3