一[（x+2)^2-4]÷x
=（x+2+2)(x+2-2)/x
=x+4.
二1/2x^2+x+1+1/2x+x
三8x^3-1/2x(a-b)^2
=(x/2)[16x^2-(a-b)^2]
=(x/2)(4x+a-b)(4x-a+b).
四a(n-1)^2-2a(n-1)+a
=a[(n-1)^2-2(n-1)+1]
=a(n-1+1)^2
=an^2.