(x²+1/2xy+1/2y²)-(x²+2/3xy-2y²)=x²+1/2xy+1/2y²-x²-2/3xy+2y²=(1-1)x²+(1/2-3/2)xy+(1/2+2)y²=-xy+5/2y²=-(-1/4)*1/2+5/2*(1/2)²=1/8+5/8=6/8=3/4