1.-a+2a^2-a^3=-a(1-2a+a^2)=-a(1-a)^2
2.16a^2b-16a^3-4ab^2=-4a(-4ab+4a^2+b^2)=-4a(2a-b)^2
3.x^4-18x^2+81=(x^2-9)^2=(x-3)^2(x+3)^2
4.x^4y^4-8x^2y^2+16=(x^2y^2-4)^2=(xy-2)^2(xy+2)^2
5.a^2-4a+4=(a-2)^2
6.3x^2+6xy+3y^2=3(x+y)^2
7.1-x+x^2/4=(1-x/2)^2
8.-4a^2x^2+12ax-9=-(2ax-3)^2
9.(a-2b)^2-14(a-2b)+49=(a-2b-7)^2
10.(x-y)^2-4(x-y-1)=(x-y)^2-4(x-y)+4=(x-y-2)^2