（a2+a+1）（a2-6a+1）+12a2
={(a^2+1)+a}{(a^2+1)-6a}+12a^2
=(a^2+1)^2-5a(a^2+1)+6a^2
=(a^2+1-2a)(a^2+1-3a)
=(a-1)^2{a-(3+√5)/2}{a-（3-√5）/2}