（a^2+a+1) (a^2-a+1),=[（a^2+1）+a] [（a^2+1)-a] = （a^2+1)^2-a ^2 = a^4+2a^2+1-a ^2= a^4+a^2+1
(2X-3Y) (2X+3Y) (4X^2+9Y^2)=(4X^2-9Y^2)(4X^2+9Y^2)=16X^4-81Y^4