A=2x^2+xy+3y^2
B=x^2-xy+2y^2
A+B=2x^2+xy+3y^2+x^2-xy+2y^2
=3x^2+5y^2
A-B=2x^2+xy+3y^2-(x^2-xy+2y^2)
=2x^2+xy+3y^2-x^2+xy-2y^2
=x^2+2xy+y^2
a=2x^2+xy+3y^2
b=x^2-xy+2y^2
a-b=x^2+2xy+y^2
a+b=3x^2+5y^2
A+B=2x^2+xy+3y^2+x^2-xy+2y^2
=3x^2+5y^2
A-B=2x^2+xy+3y^2-(x^2-xy+2y^2)
=2x^2+xy+3y^2-x^2+xy-2y^2
=x^2+2xy+y^2