Tính tổng A + B và hiệu A - B a) A = 2x + 3y và B = 2x - y b) A = $x^{2}$$y^{}$ + $\frac{3}{2}$$xy^{3}$ - $\frac{11}{2}$$x^{3}$$y^{2}$ + $x^{3}$ và B = $\frac{1}{2}$$xy^{3}$ - $x^{2}$$y^{}$ + $\frac{9}{2}$$x^{3}$$y^{2}$

`a)` Ta có:

`A+B=(2x + 3y) + (2x - y)`

`= 2x + 3y + 2x - y`

`= (2x + 2x )+ (3y - y)`

`= 4x + 2y`

`A -B = (2x + 3y) - (2x - y)`

`= 2x + 3y - 2x + y

` = (2x  - 2x )+ (3y+ y)`

`= 0 + 4y`

`=4y`

`b)` Ta có: 

`A+B = (x^2 y + 3/2 xy^3 - 11/2 x^3 y^2 + x^3) + ( 1/2 x y^3 - x^2y + 9/2 x^3 y^2 )`

`= x^2 y + 3/2 xy^3 - 11/2 x^3 y^2 + x^3 + 1/2 x y^3 - x^2y + 9/2 x^3 y^2`

 `= (x^2 y - x^2 y)+( 3/2 xy^3  + 1/2 x y^3) - (11/2 x^3 y^2 + 9/2 x^3 y^2)+ x^3 `

`= 2x y^3 -  x^3 y^2 + x^3`

`A-B = (x^2 y + 3/2 xy^3 - 11/2 x^3 y^2 + x^3) - ( 1/2 x y^3 - x^2y + 9/2 x^3 y^2 )`

`= x^2 y + 3/2 xy^3 - 11/2 x^3 y^2 + x^3 - 1/2 x y^3 + x^2y - 9/2 x^3 y^2`

`= x^2 y + 3/2 xy^3 - 11/2 x^3 y^2 + x^3 - 1/2 x y^3 + x^2y + 9/2 x^3 y^2`

`= (x^2 y + x^2 y)+( 3/2 xy^3  - 1/2 x y^3) - (11/2 x^3 y^2 + 9/2 x^3 y^2)+ x^3`

 `= 2x^2 y + x y^3 - 10x^3 y^2 + x^3`