①1/2a²b²+2+2ab=1/2(a²b²+4+4ab)=1/2(ab+2)²②x³-4x=x(x²-4)=x(x+2)(x-2)③6(x-y)-9-(x-y)²=-[(x-y)²-6(x-y)+9]= -(x-y-3)²④m²-2mn+n²-16=(m-n)...