a^n+2-6a^n+5a^n-2
=a^(n-2)(a^4-6a^2+5)
=a^(n-2)(a^2-1)(a^2-5)
=a^(n-2)(a^2-5)(a+1)(a-1)(有理数范围)
=a^(n-2)(a+√5)(a-√5)(a+1)(a-1)(实数范围)