Đáp án+Giải thích các bước giải:

`1) (3x^{2}-4).(x+3y)`

`=3x^{2}.(x+3y)-4.(x+3y)`

`=3x^{3}+9x^{2}y-4x-12y`

`2)`

`(x+3).(x^{2}+3x)`

`=x.(x^{2}+3x)+3.(x^{2}+3x)`

`=x^{3}+3x^{2}+3x^{2}+9x`

`=x^{3}+6x^{2}+9x`

`3)`

`(xy-1).(xy+5)`

`=xy.(xy+5)-1.(xy+5)`

`=x^{2}y^{2}+5xy-xy-5`

`=x^{2}y^{2}+4xy-5`

`4)`

`(3x+5y).(2x-7y)`

`=3x.(2x-7y)+5y.(2x-7y)`

`=6x^{2}-21xy+10xy-35y^{2}`

`=6x^{2}-11xy-35y^{2}`

`5)`

`-(x-1).(-x^{2}+2y)`

`=(x-1).(x^{2}-2y)`

`=x.(x^{2}-2y)-1.(x^{2}-2y)`

`=x^{3}-2xy-x^{2}+2y`

`6)`

`(-x^{2}+2y).(x^{2}+2y)`

`=-(x^{2}-2y).(x^{2}+2y)`

`=-[(x^{2})^{2}-(2y)^{2}]`

`=-x^{4}+4y^{2}`