8ax²--4ax+½a
=1/2a(16x²-8x+1)
=1/2a(4x-1)²
½x²--xy+½y²
=1/2(x²-2xy+y²)
=1/2(x-y)²
（a²+2b）²--（2b+1）²
=(a+2b+2b+1)(a+2b-2b-1)
=(a+4b+1)(a-1)