`(x+y)^3-x^3-y^3`
`=(x+y)^3-(x^3+y^3)`
`=(x+y)^3-(x+y)(x^2-xy+y^2)`
`=(x+y)[(x+y)^2-x^2+xy-y^2]`
`=(x+y)(x^2+2xy+y^2-x^2+xy-y^2)`
`=(x+y).3xy`
a) Ta có: \(\left(x+y\right)^3-x^3-y^3\)
\(=x^3-x^3+y^3-y^3+3x^2y+3xy^2\)
\(=3xy\left(x+y\right)\)
(x+y)3−x3−y3(x+y)3-x3-y3
=(x+y)3−(x3+y3)=(x+y)3-(x3+y3)
=(x+y)3−(x+y)(x2−xy+y2)=(x+y)3-(x+y)(x2-xy+y2)
=(x+y)[(x+y)2−x2+xy−y2]=(x+y)[(x+y)2-x2+xy-y2]
=(x+y)(x2+2xy+y2−x2+xy−y2)=(x+y)(x2+2xy+y2-x2+xy-y2)
=(x+y).3xy