a=x²-2xy+y
b=-x²+xy-y
c=2x+3y-1
d=-x-y+1
a. a+b=x²-2xy+y-x²+xy-y=-xy
b. c-d=x+2y
c. a+b+c-d=-xy+x+2y (de la punctele anterioare)
d. c+b-2x²=2x+3y-1-x²+xy-y-2x²=-3x²+2x+2y-1
e. a-b-c+d=x²-2xy+y+x²-xy+y-2x-3y+1-x-y+1=
=2x²-3xy-2y-3x+2
10.2x-3y+4
a. (2x-3y) +4
b.2x+(-3y)+1+2+1
c. (2x+4)-3y
