Đáp án:

`a):B=3ab^2+4b^2−3.` 

`b):B=4xy−8y^5.`

Giải thích các bước giải:

`a)`Ta có:
`2ab^2+5b^2-B=b^2-ab^2+3`

`=>2ab^2+5b^2−(b^2−ab^2+3)=B`

`=>B=2ab^2+5b^2−b^2+ab^2−3`

`=>B=(2ab^2+ab^2)+(5b^2−b^2)−3`

`=>B=3ab^2+4b^2−3.`

Vậy, `B=3ab^2+4b^2−3.`

`b)`Ta có:
`B+xy+y^5+3x^3=5xy-7y^5+3x^3`

`=>B=5xy−7y^5+3x^3−(xy+y^5+3x^3)`

`=>B=5xy−7y^5+3x^3−xy−y^5−3x^3`

`=>B=(5xy−xy)+(−7y^5−y^5)+(3x^3−3x^3)`

`=>B=4xy−8y^5.`

Vậy, `B=4xy−8y^5.`