Đáp án:

Giải thích các bước giải:

 Bài `8` : 

` P(x) + Q(x) = ( x^2y + x^3 - xy^2 + 3 ) + ( x^3 + xy^2 - xy - 6 ) `

` P(x) + Q(x) = x^2y + x^3 - xy^2 + 3 + x^3 + xy^2 - xy - 6 `

` P(x) + Q(x) = x^2y + ( x^3 + x^3 ) + ( xy^2 - xy^2 ) + ( 3 - 6 ) `

` P(x) + Q(x) = x^2y + 2x^3 - 3 `

``

Bài `9` :

` M + N = ( 3xyz - 3x^2 + 5xy - 1 ) + ( 5x^2 + xyz - 5xy + 3 - y ) `

` M + N = 3xyz - 3x^2 + 5xy - 1 + 5x^2 + xyz - 5xy + 3 - y `

` M + N = ( 3xyz + xyz ) + ( 5x^2 - 3x^2 ) + ( 5xy - 5xy ) ( - 1 + 3 ) - y`

` M + N = 4xyz + 2x^2 - y + 2 `

``

` M - N = ( 3xyz - 3x^2 + 5xy - 1 ) - ( 5x^2 + xyz - 5xy + 3 - y ) `

` M - N = 3xyz - 3x^2 + 5xy - 1 - 5x^2 - xyz + 5xy - 3 + y `

` M - N = ( 3xyz - xyz ) + ( - 3x^2 - 5x^2 ) + ( 5xy + 5xy ) - ( 1 + 3 ) + y `

` M - N = 2xyz - 8x^2 + 10xy - 4 + y `

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` N - M = ( 5x^2 + xyz - 5xy + 3 - y ) - ( 3xyz - 3x^2 + 5xy - 1 ) `

` N - M = 5x^2 + xyz - 5xy + 3 - y - 3xyz + 3x^2 - 5xy + 1 `

` N - M = ( 5x^2 + 3x^2 ) + ( xyz - 3xyz ) - ( 5xy + 5xy ) + ( 3 + 1 ) - y `

` N - M = 8x^2 - 2xyz - 10xy + 4 - y `