Đáp án+Giải thích các bước giải:

Bài `6:`

`1)`

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

`=5x^{3}y-1/4x^{3}y-x^{3}y-3/8x^{3}y`

`=x^{3}y.(5-1/4-1-3/8)`

`=x^{3}y.(4-2/8-3/8)`

`=x^{3}y.(32/8-5/8)`

`=27/8 x^{3}y`

Vậy `C-D=27/8x^{3}y`

`2)`

`D=-C+B`

`=>B-C=D`

`=>B=D+C`

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

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

`=>B=x^{3}y.(6+3/8-2/8)`

`=>B=x^{3}y.(48/8+1/8)`

`=>B=49/8 x^{3}y`

Vậy `B=49/8 x^{3}y`

Bài `7:`

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

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