（a²-a）² - 8（a-2）（a+1）-1=(a²-a)²-8(a²-a-2)-1=(a²-a)²-8a²+8a+16-1
=(a²-a)²-8a(a²-1)+15=[(a²-a)-3][(a²-a)-5]
=(a²-a-3)(a²-a-5)=[(a-1/2)²-1/4-3][(a-1/2)²-1/4-5]
=[(a-1/2)²-13/4][(a-1/2)²-21/4]
=(a-1/2+√13/2)(a-1/2-√13/2)(a-1/2+√21/2)(a-1/2-√21/2)