（x^2+1）(x+1)(x-1)
=(x^2+1)(x^2-1)
=x^4-1
(2a+1/2)(2a-1/2)(4a^2+1/4）
=(4a^2-1/4)(4a^2+1/4)
=16a^4-1/16
（x+3)(x-3)-2(2-x)(2+x)
=x^2-9-2(4-x^2)
=x^2-9-8+2x^2
=3x^2-17
(2x+3y)(3x-2y)-(x-2y)(x+2y)
=6x^2+5xy-6y^2-x^2+4y^2
=5x^2+5xy-2y^2
(a^2-4）（a^2-5）+（3a+5)(3a-5）
=a^4-9a^2+20+9a^2-25
=a^4-5
[(2a-1)(2a+1)+(a+2)(a-2)-4a^2]（a^2+5)
=(4a^2-1+a^2-4-4a^2)(a^2+5)
=(a^2-5)(a^2+5)
=a^4-25