Chứng minh: (a+b+c)^3=a^3+b^3+c^3+3x(a+b)x(b+c)x(c+a) Cm: ( a+b+c)^3 – a^3-b^3-c^3=3.(a+b).(b+c).(c+a)

`(a+b+c)^3=a^3+b^3+c^3+3(a+b)(b+c)(c+a)(\text{sai đề})`

$VT=( a+b+c)^3 – a^3-b^3-c^3-3abc=\\a^3+b^3+c^3+3(a+b)(a+c)(b+c)-a^3-b^3-c^3\\=3.(a+b).(b+c).(c+a)=VP$