a^4+2a^3+3a^2+2a+1
=a^4+2a^3+a^2+2a^2+2a+1
=a^2(a^2+2a+1)+2a(a+1)+1
=a^2(a+1)^2+2a(a+1)+1
=[a(a+1)+1]^2
=(a^2+a+1)^2