如何用平方差公式计算下列题目?

(1/2 x -y)(x+2y)
=1/2(x-2y)(x+2y)
=1/2(x^2-4y^2)
=1/2x^2-2y^2
ab(1/3 ab -b^2)(1/3 a^2 +ab)
=ab*b*(1/3a-b)*a*(1/3a+b)
=a^2b^2(1/3a-b)(1/3a+b)
=a^2b^2(1/9a^2-b^2)
2(3+1)*(3^2 +1)(3^4 +1)
=[(3-1)(3+1)]*(3^2 +1)(3^4 +1)
=(3^2 -1)(3^2 +1)(3^4 +1)
=(3^4 -1)(3^4 +1)
=(3^8 -1)