a^4-5a^2+4=(a^2-1)(a^2-4)=(a-1)(a+1)(a-2)(a+2)
a^4-5a^2+4=(a^2-1)(a^2-4) =(a-1)(a+1)(a-2)(a+2)
(a+2)(a-2)(a+1)(a-1)