4題f.2數唔識><(20點)好急

2(3p+4)(3p-4)
=2[(3p)^2-4^2] <-------(a+b)(a-b)=a^2-b^2 identity
=2(9p^2-16)
=18p^2-32 //

-3(8s+3t)(3t-8s)
=3(8s+3t)(8s-3t) <------------ -(b-a)=a-b
=3[(8s)^2-(3t)^2]
=3(64s^2-9t^2)
=192s^2-27t^2 //

-(-9h-6k)(2k-3h)
=3(3h+2k)(2k-3h) <---------- take out -3 from -9h-6k => -3(3h+2k)
=3(2k+3h)(2k-3h)
=3[(2k)^2-(3h)^2]
=3(4k^2-9h^2)
=12k^2-27h^2 //

8(a/2+b/4)(a/2-b/4)
=8[(a/2)^2-(b/4)^2]
=8(a^2 / 4 - b^2 / 16)
=2a^2 -(b^2 / 2) //