（x²+1/2xy+1/2y²）-（x²+2/3xy-2y²）,其中x=-1/4,y=1/2

（x²+1/2xy+1/2y²）-（x²+2/3xy-2y²）,其中x=-1/4,y=1/2
（x²+1/2xy+1/2y²）-（x²+2/3xy-2y²）,其中x=-1/4,y=1/2

（x²+1/2xy+1/2y²）-（x²+2/3xy-2y²）,其中x=-1/4,y=1/2
(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