-x³+x²-¼x=-¼(4x²-4x+1)=-¼(2x-1)²
-x³+x²-¼x=-(4x³-4x²+x)/4=-x(4x²-4x+1)/4=-x(2x-1)²/4
-x³+x²-¼x=x（-x²+x-1/4）=-x(x-1/2)²
-x³+x²-¼x=x(-x^2+x-1/4)=-x(x-1/2)^2
