A cura di: Alfredo Bochicchio

$2xy+(-1/2a^2)+(-1/3xy)-(-2xy)$
$2xy-1/2a^2-1/3xy+2xy$
$(2+2-1/3)xy-1/2a$
$(4-1/3)xy-1/2a$
$(12-1)/3xy-1/2a$
$11/3xy-1/2a^2$