Đáp án:

Giải thích các bước giải:

 Bài `5` :

`1)`

` M - N = ( - 3x^3y + 5/7x^3y ) - ( - 2/7x^3y + 2x^3y ) `

` M - N = - 3x^3y + 5/7x^3y + 2/7x^3y - 2x^3y `

` M - N = ( - 3x^3y - 2x^3y ) + ( 5/7x^3y + 2/7x^3y ) `

` M - N = - 5x^3y + x^3y `

` M - N = - 4x^3y `

``

`2)`

` M = - N + A `

`-> A = M + N `

` A = ( - 3x^3y + 5/7x^3y ) + ( - 2/7x^3y + 2x^3y ) `

` A = - 3x^3y + 5/7x^3y - 2/7x^3y + 2x^3y `

` A = ( - 3x^3y + 2x^3y ) + ( 5/7x^3y - 2/7^3y ) `

` A = - x^3y + 3/7x^3y `

` A = - 4/7x^3y `

``

Bài `6` :

`1)`

` C - D = ( 5x^3y - 1/4x^3y ) - ( x^3y + 3/8x^3y ) `

` C - D = 5x^3y - 1/4x^3y - x^3y - 3/8x^3y `

` C - D = ( 5x^3y - x^3y ) - ( 1/4x^3y + 3/8x^3y ) `

` C - D = 4x^3y - 5/8x^3y `

` C - D = 27/8x^3y `

``

`2)`

` D = - C + B `

`-> B = D + C `

` B = ( x^3y + 3/8x^3y ) + ( 5x^3y - 1/4x^3y ) `

` B = x^3y + 3/8x^3y + 5x^3y - 1/4x^3y `

` B = ( x^3y + 5x^3y ) + ( 3/8x^3y - 1/4x^3y ) `

` B = 6x^3y + 1/8x^3y `

` B = 49/8x^3y `

``

Bài `7` :

`1)`

` ( x + y ) + ( x - y ) `

`= x + y + x - y `

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

`= 2x + 0 `

`= 2x `

``

`2)` 

` ( x + y ) - ( x - y ) `

`= x + y - x + y `

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

`= 0 + 2y `

`= 2y `