A+B+C=(42-5xy+3y2)+(3x2+2xy+y2)+(-x+3xy+2y2)
A+B+C=(-5xy+2xy+3xy)+(3y2+y2+2y2)+3x2-x+42
A+B+C=6y2+3x2-x+42
\(C-A-B=\left(-x^2+3xy+2y^2\right)-\left(4x^2-5xy+3y^2\right)-\left(3x^2+2xy+y^2\right)\)
\(=-x^2+3xy+2y^2-4x^2+5xy-3y^2-3x^2-2xy-y^2\)
\(=\left(-x^2-4x^2-3x^2\right)+\left(3xy+5xy-2xy\right)+\left(2y^2-3y^2-y^2\right)\)
\(=-8x^2+6xy-2y^2\)
Các câu con lại bạn tự làm nha
