Răspuns :
(x+y-1)² = x²+y²+1² + 2xy + 2*(-1)y + 2*x*(-1) = x²+y²+1+2xy-2y-2x
Formula : (a+b+c)² = a²+b²+c² + 2ab + 2bc + 2ac
(x+y-1)²=
(x+y-1)(x+y-1)=
x(x+y-1)+y(x+y-1)-(x+y-1)=
x²+xy-yx+y²-y-x-y+1=
x²+(xy+xy)+(-x-x)+y²+(-y-y)+1=
x²+2xy-2x+y²-2y+1
(x+y-1)(x+y-1)=
x(x+y-1)+y(x+y-1)-(x+y-1)=
x²+xy-yx+y²-y-x-y+1=
x²+(xy+xy)+(-x-x)+y²+(-y-y)+1=
x²+2xy-2x+y²-2y+1