1、1/4x+x³-x²
=1/4x(4x²-4x+1)
=1/4x(2x-1)²
2、2mx²-4mxy+2my²
=2m(x²-2xy+2y²)
=2m(x-y)²
3、1/3x²-2xy+3y²-2
=1/3(x²-6xy+9y²)-2
=1/3(x-3y)²-2
=1/3[x-3*(1/3x-1)]²-2
=1/3(x-x+3)²-2
=3-2
=1
4、s²-2st+t²
=（s-t)²
=(t+1/2-t)²
=1/4