$6)$ $F=(x+2y)²-(x-3y)²+(2y-3x)²+(x-y)²$
$Thay$ x=$\frac{-1}{2}$ và $y=-1$ $vào$ $F$
$F=$[$\frac{-1}{2}$+2.(-1)]²-[$\frac{-1}{2}$-3.(-1)]²+[(2.-1)-(3.$\frac{-1}{2}$)]²+[$\frac{-1}{2}$-(-1)]²
$F=$$\frac{25}{4}$- $\frac{25}{4}$+$\frac{1}{4}$+$\frac{1}{4}$
$F=$$\frac{1}{2}$.