（-2/3x-3/4y )^2=(2x/3+3y/4)² =(2x/3)²+2×2x/3×3y/4+(3y/4)²=4x²/9+xy+9y²/16
原式=(2/3x+3/4y)^2=(2/3x)^2+(3/4y)^2+2(2/3x)(3/4y)=4/9x^2+9/16y^2+1/xy
4/3x^2+xy+9/16y^2
